Lookup on real field fields are not supported in standard ax.
kindly proceed to following location
AOT\\Classes\\SysTableLookup::performFormLookup()
add a new case statement as following
case classnum(FormRealControl):
callingRealControl = callingControl;
callingRealControl.performFormLookup(this.formRun());
break;
//on top of this method also add following line of code
FormRealControl callingRealControl;