While testing a theme on my phone I came across the previous message. It seems to occur sporadically with no one particular action causing a break although I have a suspicion it has something to do with layers and images, dragging and dropping from one part of the preview to another. Some breaks caused elements to appear white. Replacing them would not fix the issue.
Luckily it was backed up and I managed to revert to a previous version. At the moment I am making a few changes at a time building and testing on the phone. If it breaks I go back and do it again.
Below is a project that produces a corrupt theme. Any help is greatly appreciated.