If a Script Debugger opens when running your application or code, the error message in the upper-left pane commonly reads:

To find the error, look at the message stack (middle-left pane). Find the lowest ExceptionalEvent>>#signalWith: message. (It's close to the top of the stack.) Not far below is the message doesNotUnderstand. Click on the message just below doesNotUnderstand; the expression that caused the error is shown, highlighted, in the description (bottom) pane.

Once you find the cause of the error, look at the variables and their values in the upper-right panes. You might need to compare the variables of several scripts to pinpoint what part of the code needs to be fixed.