Fix null pointer error of guidelines

[Implementist]

1. Bugfix：When calling canvas.toDataURL after initCenteringGuidelines(canvas) or initAligningGuidelines(canvas) it will cause a “Reference clearRect of null” error.

[asturur]

Hi @Implementist can you share how this happen maybe in a jsfiddle? i wonder if the bug is somewhere else.
Your fix is ok, i just want to understand it better

[Implementist]

Hi @Implementist can you share how this happen maybe in a jsfiddle? i wonder if the bug is somewhere else.
Your fix is ok, i just want to understand it better

Thanks for your reply. I tried to reproduce this error with simple objects like Rect, Cicle, Text and Line on codepen but got failed. I consider if this problem just happen in more complex canvas (a resume etc.).

The link of my codepen is url

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[ShaMan123]

This was a symptom caused by a race condition between dispose and render - fixed (hopefully for good) by #8220