public class myaccountcontroller
{
  public String accId {get;set;}

  public myaccountcontroller(ApexPages.StandardController sc)
  {
    accId = ApexPages.CurrentPage().getparameters().get('id');
    doCheck();
  }

  public PageReference doCheck()
  {
     Account acc = [SELECT Id,Name FROM Account WHERE Id=:accId];

     if(acc.Type =='')
     {
        PageReference mainHome= Page.Custom_Error_Page;
        mainHome.setRedirect(true);
        return mainHome;
     }
  }
}

<apex:page standardcontroller="Account" extensions="myaccountcontroller">
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</apex:page>