[Bug]: getting Frame geometry causes error sometimes
### What happened?

Sentry issue: https://tldraw.sentry.io/issues/6091614216
Likely PR: https://github.com/tldraw/tldraw/pull/4979

I'm stumped at the moment about how to reproduce though. I think it's upon page load, where the editor doesn't have access to the DOM yet maybe? 
Distilled stack trace is (you can see in Sentry too):
→ Editor instantiation
→ `notVisibleShapes.ts `
→ `Editor.getShapeMaskedPageBounds`
→ `Editor.getShapeGeometry`
→ `FrameShapeUtil.getGeometry`
→ `getFrameHeadingInfo`
→ `TextManager.measureElementTextNodeSpans` (crash, no rect available)

cc @SomeHats @steveruizok if you have ideas. I guess we can never safely have TextManager within getGeometry, is that true?
