This is the obsolete method to create a property page. You have this exception because ‘handler’ parameter is nothing. You need to use SldWorks::GetRunningCommandInfo method for your needs which will return the currently opened Property Manager Page. You can also capture the DSldWorksEvents_CommandOpenPreNotifyEventHandler to handle the moment when the property page is opened.
Thanks a lot Artem,
It really helped me very much
Only change I have done is that I have used swApp_OnIdleNotify() event
along with swApp_CommandOpenPreNotify event
Thanks again & have great new year