1. Have the bin-directory of a Qt build in PATH.
   I used Qt 5.15.2 as well as Qt 6.2.0 beta2.
2. Start Creator built on Qt 6.2.0 beta2 with empty settings and open Creator 5.0 as a project:

   ~/dev/g++-9.3.0/creator-5.0-6.2.0/bin/qtcreator ~/dev/src/creator-5.0/qtcreator.pro -tcs
   

   The sources are on 5.0 branch, revision 71f2e7566ddb7e6dfe74bbf4625419a5899155fe. These also are the sources I built Creator from.
   Creator will show you the "Configure Project" screen.

3. Click "Configure Project".
   Creator often crashes, although not always.

I could only reproduce this with Creator built on Qt 6.2 (using qt-cmake), not with a build on 5.15.2 (using qmake).