Exception: System.NullReferenceException (4329256)

After definining the columns and running the query with two joined tables, receive the error as given below for Toad for Data Analysts:

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Exception Information
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Exception: System.NullReferenceException
Message : Object reference not set to an instance of an object.
Stack Trace:
at Quest.Toad.Gui.MainForm.FindOpenedToolWin(Type type)
at Quest.Toad.DatabaseBrowser.ObjectPaletteControl.get_ActivePalette()
at Quest.Toad.QueryBuilder.QueryBuilderControl.get_SelectedDatabase()
at Quest.Toad.QueryBuilder.QueryBuilder.get_SelectedDatabase()
at Quest.Toad.ExplainPlan.ExplainPlanControl.GetSQLText()
at Quest.Toad.ExplainPlan.ExplainPlanControl.DoExplain(Object document)