Hi All,
Can someone please let me know how  i can change the error being displayed from Uncaught Action failed: c:salesOwnerChange$controller$submitSelected [Cannot read property 'length' of undefined] to some custom error message. I wanted to show an error when the submit for approval is clicked without any accounts being selected from the lightning component(which means slect list passed to update accounts would be null)
Below is my code:

CustomLookupcontroller.apxc:
public class customLookupController {
    @AuraEnabled
    public static List <sObject> fetchLookUpValues(String searchKeyWord, String ObjectName) 
    {
        system.debug('searchkeyword:::'+searchKeyWord);
        system.debug('ObjectName-->' + ObjectName);
        String searchKey = searchKeyWord + '%';
        
        List <sObject> returnList = new List <sObject> ();
      
        String sQuery =  'select id, Name from ' +ObjectName + ' where Name LIKE: searchKey order by createdDate DESC limit 5';
        List <sObject> lstOfRecords = Database.query(sQuery);
        system.debug('lstOfRecords:::'+ lstOfRecords);
        for (sObject obj: lstOfRecords) 
        {
            returnList.add(obj);
        }
        return returnList;
    }
    
    @AuraEnabled
    public static List<Account> ownerList(Id user)
    {
        List<Account> accList = [SELECT Name FROM Account WHERE OwnerId = :user ORDER BY Name asc];
        return accList;
    }
    
    @AuraEnabled
    public static void updateAccounts(List<Id> selectList, Id fromId, Id userId, String requestDate, String effectiveDate)
    {
        integer lstsize = selectList.size();
        if(lstsize!=0)
        {
         system.debug('its entering here'); 
        Change_Request__c req = new Change_Request__c();
        system.debug('fromId:::'+fromId);
        system.debug('userId:::'+userId);
        system.debug('selectList:::'+ selectList);
        system.debug('selectListSize:::'+selectList.size());
        List<Account> updateList = [SELECT Id, Name, To_Salesperson__c FROM Account WHERE Id in :selectList];
        List<User> fromUser = [select ManagerId from User where Id = :fromId];
        List<User> ToUser = [select ManagerId from User where Id = :userId];
        
        req.AccountForChange__c = '';
        for(Account acc : updateList)
        {
            system.debug('acc::' + updateList);
            req.AccountForChange__c = req.AccountForChange__c + acc.Name + '\n';
        }
        for(User acc1 : fromUser)
        {
            system.debug('acc1::' + fromUser);
            req.From_Salesperson_Manager__c = acc1.ManagerId ;
        }
        for(User acc2 : ToUser)
        {
            system.debug('acc2::' + ToUser);
            req.To_Salesperson_Manager__c = acc2.ManagerId  ;
        }
       
        Date reqt = Date.valueOf(requestDate);
        Date efft = Date.valueOf(effectiveDate);
        System.debug(reqt);
        System.debug(efft);
        
        req.RecordTypeId = '012f40000005PNY';
        req.To_Salesperson__c = userId;
        req.From_Salesperson__c = fromId;
        req.Status__c='Pending';
       
        req.Date_Requested__c = reqt;
        req.Effective_Date_of_Change__c = efft;
        insert req;
        }
}
    
    @AuraEnabled
    public static List<Account> ownerList2(Id account)
    {
        system.debug('account:::'+account);
        if(account==null)
        {
            List<Account> nullList = [SELECT Name FROM Account WHERE Broker__c = :null];
            return nullList;
        }
        else
        {
            List<Account> accList = [SELECT Name FROM Account WHERE Broker__c = :account ORDER BY NAME asc];
            return accList;
        }
    }
}

Lightning component:
<aura:component controller="customLookupController" implements="force:appHostable,flexipage:availableForAllPageTypes,flexipage:availableForRecordHome,force:hasRecordId,forceCommunity:availableForAllPageTypes,force:lightningQuickAction" access="global">
    <aura:attribute name="fromSalesperson" type="sObject" default="{}" />
    <aura:attribute name="toSalesperson" type="sObject" default="{}" />
    <aura:attribute name="searchResult" type="List" default="{}"/>
    <aura:attribute name="request" type="date" />
    <aura:attribute name="effective" type="date" />
 
    <table>
        <tr>
            <td style="padding:15px"><c:customLookup objectAPIName="user" IconName="standard:user" selectedRecord="{!v.fromSalesperson}" label="From Salesperson"/></td>
              <td style="padding:15px"><c:customLookup objectAPIName="user" IconName="standard:user" selectedRecord="{!v.toSalesperson}" label="To Salesperson"/></td> 
        </tr>
        <tr>
            <td style="padding:15px"><lightning:input aura:id="requestDate" value="{!v.request}" name="dateRequested" type="date" label="Date Requested"/></td>
            <td style="padding:15px"><lightning:input aura:id="effectiveDate" value="{!v.effective}" name="effectiveDateOfChange" type="date" label="Effective Date of Change"/></td>
        </tr>
    </table>
    
    <div style="padding:15px">
        <lightning:button name="submit" label="Search" onclick="{!c.submit}" />
    </div>
    
    <div style="padding-left:15px; padding-right:15px">
        <table class="slds-table slds-table--bordered slds-table--cell-buffer">
          <thead>
             <tr class="slds-text-title--caps">
                <th style="width:3.25rem;" class="slds-text-align--right">
                   <div class="slds-form-element">
                      <div class="slds-form-element__control">
                         <label class="slds-checkbox">
                            <!--header checkbox for select all-->
                            <ui:inputCheckbox aura:id="box3" change="{!c.selectAll}"/>
                            <span class="slds-checkbox--faux"></span>
                            <span class="slds-form-element__label text"></span>
                         </label>
                      </div>
                   </div>
                </th>
                <th>
                   <span class="slds-truncate" title="Name">Accounts</span>      
                </th>
             </tr>
          </thead>
          <!--table body start, 
             Iterate contact list as a <tr>
             -->
          <tbody>
             <aura:iteration items="{!v.searchResult}" var="acc">
                <tr>
                   <td scope="row" class="slds-text-align--right" style="width:3.25rem;">
                      <div class="slds-form-element">
                         <div class="slds-form-element__control">
                            <label class="slds-checkbox">
                               <ui:inputCheckbox text="{!acc.Id}" aura:id="boxPack" value=""/>
                               <span class="slds-checkbox--faux"></span>
                               <span class="slds-form-element__label text"></span>
                            </label>
                         </div>
                      </div>
                   </td>
                   <td scope="row">
                      <div class="slds-truncate" title="{!acc.Name}"><a>{!acc.Name}</a></div>
                   </td>
                </tr>
             </aura:iteration>
          </tbody>
       </table>
       </div>
       <div style="padding:15px">
           <lightning:button name="submitForApproval" label="Submit For Approval" onclick="{!c.submitSelected}"/>
       </div>
</aura:component>
Lightning component controller/salesownerchangecontroller.js:
({
    submit : function(component, event, helper) 
    {
        var userId = component.get("v.fromSalesperson");
        var action = component.get("c.ownerList");
        action.setParams({
            'user': userId.Id
        });
        action.setCallback(this, function(response) {
            var state = response.getState();
            if (state === "SUCCESS") 
            {
                var storeResponse = response.getReturnValue();
                component.set("v.searchResult", storeResponse);
            }
 });
        $A.enqueueAction(action);
    },
    
      selectAll: function(component, event, helper) 
    {
          var selectedHeaderCheck = event.getSource().get("v.value");
          var getAllId = component.find("boxPack");
        system.debug('boxPack:::'+ boxPack);
          if (selectedHeaderCheck == true) 
          {
               for (var i = 0; i < getAllId.length; i++) 
            {
                component.find("boxPack")[i].set("v.value", true);
                component.set("v.selectedCount", getAllId.length);
               }
          } 
         else 
        {
               for (var i = 0; i < getAllId.length; i++) 
            {
                component.find("boxPack")[i].set("v.value", false);
                component.set("v.selectedCount", 0);
               }
          }
     },
    
    submitSelected: function(component, event, helper) 
    {
        var selId = [];
          var getAllId = component.find("boxPack");
        var getRequest = component.get("v.request");
        var getEffective = component.get("v.effective");
        component.set("v.request", getRequest);
        component.set("v.effective", getEffective);
          for (var i = 0; i < getAllId.length; i++) 
        {
               if (getAllId[i].get("v.value") == true) 
            {
                selId.push(getAllId[i].get("v.text"));
               }
          }
          helper.submitSelectedHelper(component, event, selId, getRequest, getEffective);
    },
})

Helper :
({
     submitSelectedHelper: function(component, event, selectRecordsIds, getRequest, getEffective) 
    {
         var userId = component.get("v.toSalesperson");
        var fromId = component.get("v.fromSalesperson");
          var action = component.get("c.updateAccounts");
        var requestDate = ""+getRequest+"";
        var effectiveDate = ""+getEffective+"";
          action.setParams({
               "selectList": selectRecordsIds,
               "userId": userId.Id,
            "fromId": fromId.Id,
            "requestDate": requestDate,
            "effectiveDate": effectiveDate
          });
          action.setCallback(this, function(response) {
           var state = response.getState();
            system.debug("state:::"+state);
           if (state === "SUCCESS") 
        {
            console.log(state);
           }
    });
          $A.enqueueAction(action);
        
        var toastEvent = $A.get("e.force:showToast");
        toastEvent.setParams({
        "title": "Success!",
        "message": "Submitted For Approval"
        });
        toastEvent.fire();
        
        var urlEvent = $A.get("e.force:navigateToURL");
        urlEvent.setParams({
          "url": "/one/one.app#/n/Sales_Owner_Change_Request"
        });
        urlEvent.fire();
    },
})


As of now in the component page if nothing is selected and we click on submit for approval i clicked: an error is thrown with following message--> Uncaught Action failed: c:salesOwnerChange$controller$submitSelected [Cannot read property 'length' of undefined]