Project with VST does not play, mixer is greyed out

Every project that I use VST to replace the default MS Basic piano, gets "corrupted" or something like that because when I close the project and open it later, the Mixer slots are greyed out and disabled. Also the project does not play, I guess because there is no sound source selected in the mixer.
But for that session when the new project is open I can change VST plugins without issues. It is after I quit and come back when issue starts.