HiDPI and immutable images

When I didn’t use the option “Supports HI-DPI” the line:
…worked well.
After usage of “Supports HI-DPI”, the application looks much better, without blur, but the code-line got NIL error problem.
I’ve managed to read somewhere on the forum, that images imported to the versions with “supports Hi-DPI” are “immutable”.
So, how to draw anything into the image?
Why the editor allows for typing such a statement?

You’ll need to make a copy of the image to draw on it.

As for the code editor, it can’t know at design time whether an image will be immutable.