When an error occurs from running code that was not evaluated using an Edit menu option, the error message commonly reads:

The message that contains the error is highlighted in the stack and the expression that caused the error is shown, highlighted, in the description pane.

After you find the cause of the error, examine message variables to see how the internal state of an object is incorrect. This is the more difficult task. You might need to compare the variables of several messages to pinpoint what part of the code needs to be fixed.