Since Debugging of the Popup is not possible directly in SAP. Cause we cannot put '/H' in the command line. So there is a way by which we can debug the Standard Popup or any Z Popup.
Create one TXT File in you system.. Lets say File name is 'Debug.Txt'.
Now Write the Below mentioned code in the .TXT file.
[FUNCTION] Command=/H Title=Debugger Type=SystemCommand
Now Save this File.
Now execute your SAP and Open any POPUP .
Steps: First go to the file 'debug.txt' and drag that file and drop that file on the POPUP
Please See below screen shot. 
Now Click the Button and you would notice that the There is one status message comes up ' Debugger switched on'. 
and when you clikc any button then the deubger will come up
