I see an error in what you posted. See the different spelling used for the variable names? If those are also the case in your actual script, it could explain the results that you are getting.
If (not isEmpty($$FontTab)
Go to Object (Object Name: $$FrontTab)
Opps! I have a typo in the posting, not the script. The posting should read:
If (not isEmpty($$FrontTab)
When you check this in the debugger, do you have the data viewer open so that you can see the value of the variable at the time the script executes? The next troubleshooting step is to compare that text to the text entered into the inspector's Name box in order to give the tab panel an object name.
Note: the tab label is NOT the object name, you have to specifically name the panel in the inspector in order to give it an object name.