That’s pretty straightforward code, and although I suggested that not checking for null might be a problem, I can’t cause the crash with sample code. I wonder if this is the same bug reported here:
It looked like the “navigate to” blocks were the problem initially (which was really weird!), but turned out to be an invisible web viewer problem, and it was an android specific problem. Any chance you’ve got an invisible web viewer somewhere on one of the screens you’re supposed to be navigating to?