Fix toggling showLine option on scatter controller

[stockiNail]

Fix #11333

[stockiNail]

@LeeLenaleee apologize, don't hate me.... can you set the label to bug in order to have a consistent release note? Thank youuu

[azmike11]

I ran into a similar problem with a scatter chart using version 4.3.0. When I tried to change the "showLine" property of my 2nd dataset (id=1) to true, and then do a update() to update the chart, The error occurred after trying to perform an update.

obj_chart["chart_1"].data.datasets[1].showLine = true;
obj_chart["chart_1"].update();

The error in the console looked something like this:

index.umd.ts:50 Uncaught TypeError: Cannot set properties of null (setting '_chart')
at ScatterController.update (index.umd.ts:50:18)
at ScatterController._update (index.umd.ts:50:18)
at On._updateDataset (index.umd.ts:50:18)
at On._updateDatasets (index.umd.ts:50:18)
at On.update (index.umd.ts:50:18)
at f_show_line (graph2.php:176:23)
at HTMLInputElement.onchange (graph2.php:209:76)
update @ index.umd.ts:50
_update @ index.umd.ts:50
_updateDataset @ index.umd.ts:50
_updateDatasets @ index.umd.ts:50
update @ index.umd.ts:50
f_show_line @ graph2.php:176
onchange @ graph2.php:209