Texture Painting is not working on ONE file only, other files work fine I'm currently using Blender 2.8

In the file i'm working on, i unwrapped a mesh (Smart UV Project) -> Add base color in the "Texture Slot", but then i am unable to paint over it.



Therefore, i tried creating a sphere in the same file and try texture painting it, it doesn't work as well. 



So i created a new file and try that again, it works. Can anyone explain why Texture painting works fine in other files but not this particular one? 
I am new to Blender so any help from you guys is very much appreciated!!
I found the solution. Maybe it is a bug that is causing the problem or i may have done something wrong, but you can fix it by just copying whole mesh to new file. And it works!