Hi! I also got the same error while compiling. I found that the header files can be generated from the Makefile by manually running the compiler_uic_make_all target.
Therefore, the workaround I have is to go to your build directory (mine is build-phoenix-Desktop_Qt_5_12_12_MSVC2017_64bit-Debug) and invoke jom.exe on Makefile.Debug:
C:\Qt\Tools\QtCreator\bin\jom\jom.exe -f Makefile.Debug compiler_uic_make_all
I would still like to figure out why the build isn’t automatically generating those headers or where it was broken.
Windows 10
Qt Creator 6.0.1
Desktop Qt 5.12.12 MSVC2017 64bit
cmake version 3.12.18081601-MSVC_2