The application crashes when adding ".umap" to the launch map at application launch. If not specify ".umap", it will be able to launch application. There appears to be a failure to parse the URL. The URL parameters indicate that it is optional.
It's a regression of 4.25 because it works in 4.24.
Following code is workaround.
Crash and show attached popup.
Not crash and open TestMap level.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-96502 in the post.