Hello!
I’m not a Wappler team, but can share a advice.
It seems, it is a bug, that already been reported Projects do not switch (occasional) 4/B13
I have met it today too.
You can close current project by “close project” in main menu. (or Alt-W combination)
After that you will be able to open another project properly, I hope.
By the way, when you will try to close project, Wappler can freeze and not response, that’s another current bug. 
In this case, you probably have to kill Wapplier via Task Manager.