qmake pre-step fails with B2Qt wizard applications

Steps to reproduce:

1. Install Qt 5.15.3 B2Qt binary and Preview Qt Creator (4.15.0-beta1)
2. Open Preview QtC and generate a qmake application with the Application Wizard

Expected result:

The pre-qmake step succeeds

Actual result:

The Issues tab shows:

Variable QMAKE_CXX.COMPILER_MACROS is not defined

Project ERROR: failed to parse default search paths from compiler output.

Error while parsing file /home/qt/RTA/quickapp/quickapp.pro. Giving up.

If one selects Build, it succeeds (calls qmake, generates .qmake.stash and the build succeeds). Does not happen with: QtC 4.14.1. And looks like this happens only with the wizard generated applications.