Xcode build failing no relevant error messages displayed
Cleaning the build folder (Cmd+Shift+K) helped me resolve the invalid reuse after initialization failure issue
Just ran into the same issue. I fixed it by double-clicking on Runner in the files overview. This showed Identity and Type on the right-hand side. If you set Project Format (under Project Document) to Xcode 12.0-compatible it works.
Hope the description helps. I only use the mac when testing Flutter builds for iOS, so I'm not an expert :p