Yes, the IDE is very much not happy if it tries to open a branch that isn’t a valid Anvil app! Although this is a situation you can only get into by messing with the git repo by hand, we should probably handle it a little more gracefully now that such messing is easier…
Your best way out here is to make your new branch temporarily point at a commit with a valid anvil app. I suggest hard-reseting it to the same commit as master, then force-pushing it, then loading the IDE and switching to another branch, then resetting and force-pushing your new branch back to where you want it.