Sorry for taking so long!
So, I FINALLY got around to checking to see if this fixed it. I made a whole new map. Followed the instructions on the wiki, have all the SRB2 configs including the new one, made sure the engine was SRB2 Kart V1.2, hit test, and...
The exact same error window. I don't get it.
...Wait, there's more this time. Afterwards, a window popped up labelled "YOU CAN'T DO THAT!" with some error text. I'll just copy and paste it here:
Error in System.Windows.Forms:
Object reference not set to an instance of an object.
at System.Windows.Forms.Control.MarshaledInvoke(Control caller, Delegate method, Object[] args, Boolean synchronous)
at System.Windows.Forms.Control.Invoke(Delegate method, Object[] args)
at System.Windows.Forms.Control.Invoke(Delegate method)
at CodeImp.DoomBuilder.Launcher.ProcessOnExited(Object sender, EventArgs eventArgs)
at System.Diagnostics.Process.OnExited()
at System.Diagnostics.Process.RaiseOnExited()
at System.Diagnostics.Process.CompletionCallback(Object context, Boolean wasSignaled)
at System.Threading._ThreadPoolWaitOrTimerCallback.WaitOrTimerCallback_Context(Object state, Boolean timedOut)
at System.Threading._ThreadPoolWaitOrTimerCallback.WaitOrTimerCallback_Context_f(Object state)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
at System.Threading._ThreadPoolWaitOrTimerCallback.PerformWaitOrTimerCallback(Object state, Boolean timedOut)
Turns out this was a crash report. After closing the window, all of ZB closed.
...I have no idea what any of this means. Someone help?