Cancelling texture selection in material editor unsets the texture instead

[grizeldi]

Cancelling texture selection in material editor unsets the texture instead

Repro steps:

1. Open a material that has texture properties set in material editor
2. Click on any texture as if you wanted to change the texture
3. Click "cancel" in the selection popup with no texture selected
4. Texture gets unset

I could only reproduce this the first time I've tried reproducing it, after manually setting a texture things work as intended until I close and reopen the material editor window. Presumably because it has a texture selected afterwards when the popup opens.

[neph1]

Maybe related to #562