In debugger, look at what's the type of the value in _journalTrans variable. It's declared as a map - you want to know which actual table it is. The code will work only with InventJournalTrans; the error suggests that _journalTrans contains something else.
If you find that the type is correct, you probably failed to re-compile X++. If you don't want to recompile the whole application, right-click JournalForm class and use Add-Ins > Compile forward. But again, first check the types; this would be a waste of if the types are actually incompatible.