Solved
You have to run (at least once) the code generation in TouchGFX 4.26.0 Designer (the Windows PC application is normaly where You have the STM32Cube repository:
C:\Users\<YourUsername>\STM32Cube\Repository\Packs\STMicroelectronics\X-CUBE-TOUCHGFX\4.26.0\Utilities\
Open the (generated) ApplicationTemplate.touchgfx.part file in TouchGFX designer and click the "Generate Code" Button.
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