Hover with adding and removing datasets may fail

[joshkel]

Expected Behavior

Assigning to chart.data.datasets and then using chart.update to refresh the chart should work.

Current Behavior

Under (roughly) the following sequence of events, an error is triggered:

• The chart is drawn with data.
• The user hovers over a data point.
• The chart is redrawn once (and only once) with no data.
• The chart is redrawn with data.

The specific error is "Cannot read properties of null (reading 'buildOrUpdateElements')" on this line.

From exploring in the debugger, the sequence of events appears to be

• chart.update() goes to replay the last event.
• For a mouse move event, that eventually calls updateHoverStyle, which tries to get the metaset for the dataset index for the hovered point.
• Since that dataset is no longer valid, getDatasetMeta creates an empty metaset that's never properly updated.

Possible Solution

Should updateHoverStyle check whether the dataset index is valid before trying to retrieve the metaset?

Alternatively, or as well, I believe that _destroyDatasetMeta has a bug; the current code should probably be

    if (meta) {
      if (meta.controller) {
        meta.controller._destroy();
      }
      delete me._metasets[datasetIndex];
    }

(I made this change in my local copy, and it appears to fix the bug.)

Or, because deleting a non-existent element should be harmless:

    if (meta.controller) {
      meta.controller._destroy();
    }
    delete me._metasets[datasetIndex];

I wasn't sure if changing updateHoverStyle fits with Chart.js's design; I can submit a PR for whatever fix seems best.

Steps to Reproduce

https://codesandbox.io/s/optimistic-perlman-705ce?file=/src/index.js

Mouse over the chart while data is visible, then wait for it to disappear and reappear.

Environment

• Chart.js version: 3.5.1
• Browser name and version: Observed in Safari and in Chrome 93

[kurkle]

Does this issue depend on animations being enabled or not?

The updateHoverStyle can be triggered by external code, so I think it would be good to validate the index.