[ Ultimate Doom Builder] – having trouble testing my map – From /r/Doom

It’s my first time with udb. I use gzdoom and the doom 2.wad. Once I pressed F9 it asked me to program the thing to run my map. I selected gzdoom and when I try to play, it says : Unable to start the test program, NullReferenceException: Object reference not set to an instance of an object.

Please help, thank you.