Tabulator: prevent JS error

[maximlt]

I have a somewhat complex app with Tabulator widgets nested in an Accordion that started to raise Javascript errors related to Panel's tabulator code. The error is raised from the last condition of the second line because this.tabulator is undefined, setData being called from the callback triggered by this.model.source.properties.data.change.

panel/panel/models/tabulator.ts

Lines 904 to 905 in 6b6eb3f

	setData(): void {
	if (this._initializing || this._building || !this.tabulator.initialized)

It seems like this was the consequence of #5786. I haven't found what exactly can cause this.tabulator to be undefined in this context, so I'm just adding a guard in the this.model.source.properties.data.change callback and that seems to do the job.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (6b6eb3f) 82.51% compared to head (5104e76) 84.03%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6223      +/-   ##
==========================================
+ Coverage   82.51%   84.03%   +1.51%     
==========================================
  Files         301      301              
  Lines       45099    45099              
==========================================
+ Hits        37214    37898     +684     
+ Misses       7885     7201     -684     

Flag	Coverage Δ
ui-tests	40.70% <ø> (+2.23%)	⬆️
unitexamples-tests	71.66% <ø> (+0.04%)	⬆️

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