<aura:component controller="SimpleValidation" implements="force:appHostable,flexipage:availableForAllPageTypes,flexipage:availableForRecordHome,force:lightningQuickAction,force:hasRecordId" access="global" >
    <aura:attribute name="CaseObj" type="Case" default="{ 'sobjectType' : 'case'}"/>  
    <lightning:button label="Redirect" onclick="{! c.handleClick}"/>
</aura:component>

({
handleClick: function(component, event, helper)
    {
        var mycase = component.get("v.CaseObj");
        var action=component.get("c.FieldCheck");
        action.setParams({
            obj: mycase,
            oId : component.get("v.recordId")
            
        });
            action.setCallback(this, function(response) {
            var state = response.getState();
            if(state === "SUCCESS")
            {
                var flag= response.getReturnValue();
                 console.log('return value of flag ' + flag);
                 if(flag == true)
                     {
                var urlEvent = $A.get("e.force:navigateToURL");
                 console.log('my case id is '+ mycase.Id);
                   
                    urlEvent.setParams({
                        "url": "/_ui/core/email/author/EmailAuthor?p3_lkid={!mycase.Id}&p2_lkid={!mycase.ContactId}&p24={!mycase.Con_Email__c};{!mycase.Con_2_Email__c};{!mycase.Con_3_Email__c}&template_id=00X60000004dTuo&p26=test@gmail.com"});
        
                        urlEvent.setParams({ "isredirect": true });
               
                urlEvent.fire();
                   }
                else if(flag == false)
                {
                    alert('Please make sure address is not empty:');  
                }
                    else{
                    alert('Check the code:');
                }
            }
            else if (state === "INCOMPLETE") {
            }
            else if (state === "ERROR") {
                var errors = response.getError();
                 if (errors) {
                    if (errors[0] && errors[0].message) {
                        console.log("Error custom message: " + errors[0].message);
                    }
                } else {
                    console.log("Unknown error");
                }
                           }
        });
        $A.enqueueAction(action);
    }
})

public with sharing class SimpleValidation {
 @AuraEnabled    
    public static boolean FieldCheck(Case obj, String oId)
    {
      try
    {
        if(obj.Con_Email__c == null){

            return false;
        }
        else{
            return true;
        }
    }
    catch(Exception e)
    {
        throw new AuraHandledException('Darn it! Something went wrong: ' + e.getMessage());   
    }    
}
}

Unable to Access Page
The value of the "p3_lkid" parameter contains a character that is not allowed or the value exceeds the maximum allowed length.
Remove the character from the parameter value or reduce the value length and resubmit.
If the error still persists, report it to our Customer Support team. Provide the URL of the page you were requesting as well as any other related information.