Error "System.ArgumentNullException' when executing any Oracle SQL in the Editor (4311627)

Running any select statement in the Editor against an Oracle database results in the following error:

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Exception Information
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Exception: System.ArgumentNullException
Message  : Value cannot be null.
Parameter name: input
Stack Trace:
   at System.Text.RegularExpressions.Regex.Match(String input)
   at Quest.Toad.Oracle.OracleExplain.FindExplainTag(CommandResult cmd)
   at Quest.Toad.Oracle.OracleExplain.AddActualExecutionPlan(CommandResult result, Boolean checkOnly)
   at Quest.Toad.Oracle.OracleToadConnection.AddActualExecutionPlan(CommandResult result, Boolean checkOnly)
   at Quest.Toad.Editor.EditorForm.AddCommandResult(ScriptResultsControl control, CommandResult cmdResult)
   at Quest.Toad.Editor.EditorForm.DoExecuteSqlStatement()
   at Quest.Toad.Editor.EditorForm.ExecuteSqlStatement()