First, I'd recommend doing the same, using a "Calculation Condition" and a custom Error Message.
You can formulate a condition that needs to be fulfilled, before the chart gets calculated - this way, the chart won't have to work hard before going "Out of Memory". Then, you can replace the default Error Message "Calculation Condition unfulfilled with a more user friendly message, like "Please select a Prospect and a Pad and ...".
So, this eliminates the need in the extra text object.
Now, in order to formulate your condition, you need to use keywords "and" / "or", to simplify your condition. For example:
(GetSelectedCount(Well_Name) < 1) or ((GetSelectedCount(Well_Prospect) < 1) and (GetSelectedCount(Well_Pad) < 1))