Hi All,
I am getting following when I am performing DML Operation
 

​System.ExternalObjectException: Salesforce System Error: 627698564-15996 (1450171494) (1450171494) : (System Code)
Error is in expression '{!Search}' in component <apex:commandButton> in page cpm_lookup1: Class.CustomAccountLookupController.performSearch: line 34, column 1
Class.CustomAccountLookupController.runSearch: line 23, column 1
Class.CustomAccountLookupController.search: line 16, column 1

This is my code

public with sharing class CustomAccountLookupController {

  public Account account {get;set;} // new account to create
  public List<externalobj1__x> results{get;set;} // search results
  public string searchString{get;set;} // search keyword

  public CustomAccountLookupController() {
    account = new Account();
    // get the current search string
    searchString = System.currentPageReference().getParameters().get('lksrch');
    runSearch();  
  }

  // performs the keyword search
  public PageReference search() {
    runSearch();
    return null;
  }

  // prepare the query and issue the search command
  private void runSearch() {
    // TODO prepare query string for complex serarches & prevent injections
    results = performSearch(searchString);               
  } 

  // run the search and return the records found. 
  private List<externalobj1__x> performSearch(string searchString) {

    String soql = 'select id,externalid from externalobj1__x';
    if(searchString != '' && searchString != null)
      soql = soql +  ' where externalid LIKE \'%' + searchString +'%\'';
    soql = soql + ' limit 25';
    System.debug(soql);
    return database.query(soql); 

  }

  // save the new account record
  public PageReference saveAccount() {
    insert account;
    // reset the account
    account = new Account();
    return null;
  }

  // used by the visualforce page to send the link to the right dom element
  public string getFormTag() {
    return System.currentPageReference().getParameters().get('frm');
  }

  // used by the visualforce page to send the link to the right dom element for the text box
  public string getTextBox() {
    return System.currentPageReference().getParameters().get('txt');
  }

}

 

<apex:page controller="CustomAccountLookupController"  
  title="Search" 
  showHeader="false" 
  sideBar="false" 
  tabStyle="Account" 
  id="pg">

  <apex:form >
  <apex:outputPanel id="page" layout="block" style="margin:5px;padding:10px;padding-top:2px;">
    <apex:tabPanel switchType="client" selectedTab="name1" id="tabbedPanel">

      <!-- SEARCH TAB -->
      <apex:tab label="Search" name="tab1" id="tabOne">

        <apex:actionRegion >  
          <apex:outputPanel id="top" layout="block" style="margin:5px;padding:10px;padding-top:2px;">
            <apex:outputLabel value="Search" style="font-weight:Bold;padding-right:10px;" for="txtSearch"/>
            <apex:inputText id="txtSearch" value="{!searchString}" />
              <span style="padding-left:5px"><apex:commandButton id="btnGo" value="Go" action="{!Search}" rerender="searchResults"></apex:commandButton></span>
          </apex:outputPanel>

          <apex:outputPanel id="pnlSearchResults" style="margin:10px;height:350px;overflow-Y:auto;" layout="block">
            <apex:pageBlock id="searchResults"> 
              <apex:pageBlockTable value="{!results}" var="a" id="tblResults">
                <apex:column >
                  <apex:facet name="header">
                    <apex:outputPanel >Name</apex:outputPanel>
                  </apex:facet>
                   <apex:outputLink value="javascript:top.window.opener.lookupPick2('{!FormTag}','{!TextBox}_lkid','{!TextBox}','{!a.Id}','{!a.externalId}', false)" rendered="{!NOT(ISNULL(a.Id))}">{!a.externalid}</apex:outputLink>     
                </apex:column>
              </apex:pageBlockTable>
            </apex:pageBlock>
          </apex:outputPanel>
        </apex:actionRegion>

      </apex:tab>

      <!-- NEW ACCOUNT TAB -->
      <apex:tab label="New Account" name="tab2" id="tabTwo">

        <apex:pageBlock id="newAccount" title="New Account" >

          <apex:pageBlockButtons >
            <apex:commandButton value="Save"/>
          </apex:pageBlockButtons>
          <apex:pageMessages />

          <apex:pageBlockSection columns="2">
            
          </apex:pageBlockSection> 
        </apex:pageBlock>

      </apex:tab>
    </apex:tabPanel>
  </apex:outputPanel>
  </apex:form>
</apex:page>

Please help me in resolving this error

Thanks in Advance