NullReferenceException when rendering annotations

[colejohnson66]

Cross post from oxyplot/oxyplot#1904

Appears to be an issue specifically with OxyPlot.Avalonia. @VisualMelon suggests two changes:

• Calling InvalidatePlot off-thread should set a dirty-flag immediately, and schedule an update on the UI thread (not just crash)
• UpdateVisuals should check the dirty-flag and perform an update if necessary before rendering

[VisualMelon]

Closing, as I believe this is fixed. Feel free to re-open/comment if you believe there is still an issue with this.