When wanting to edit, screen goes blank

Hi Everybody

I have just created a new app (have done apps before) and just upgraded to pro. When I click on the app to edit, it appears on the screen for a second and then the whole screen goes white. Never happened before. This app is bigger than my previous ones though. Does that make a difference?

Only in UI editor or real app as well?

Only in editor but then have just tried it on a different computer and it opens fine. Not sure what that is about seeing as first computer is new and second is old

I have seen it in the editor but not in a real app.