Why have all files in Flutter project red? And why I see a blank screen when I run the code?

Question

I have made a project with Flutter and everything was going fine until I closed my project and when I opened it: I see that all files have turned red and I cannot view anything on the emulator but a white screen (but the first page works just fine).

This is the image of all files turning red.

Then, when I run one of the pages, I see a blank screen and a Failed Assertion error: hasSize I'm unaware of these problems.

Can someone help me?

Thanks in advance!