An Illegal Address error indicates that the software could not find a field. The most common cause is that a window was closed while a process was running. A second cause is that the code is looking for a field that does not exist because you do not have security access to a customized window.
Here are several examples:
Illegal address for field Reason Code in script DisassembleFromBOM
Illegal address for field 'TrigTag' in script 'wAlertHandler'
Illegal address for field 'Item Number' in script '[Not Found]'
Illegal address for field 'iRecordNoteShowHide' in script 'wIVItemInquiry_DisplayItemID_AORG'
Illegal address for field 'cbTreeRefreshed' in script 'Done'
Illegal address for field 'Based On Invoice Total' in script 'Tax_Calc_Line'
Firstly, this is a software error that needs to be addressed. The code should make sure that needed windows are, or remain, open while it is running. If the error is coming from a WilloWare module, we will quickly address the issue and upload a new release.
Secondly, you may be able to prevent the error by making sure the GP has completed a task before closing windows.
Lastly, if the error occurs when opening a window, it may be a security issue. Please check that you have security permissions to modified or alternate version of the window you are opening. For example, if you are opening Item Maintenance and receive an Illegal Address For Field error, check if there is a modified/alternate version of Item Maintenance and try using that.