Fix for getLabelAndValue on null controller

I encountered chartjs#11315 under the following circumstances:

1. Position the cursor over the chart area, such that it causes a
   tooltip to be shown.
2. Move the cursor out of the chart area, such that the tooltip remains
   visible.
3. Cause the chart contents to be changed, such that the dataset
   referenced by the active tooltip element is no longer valid.
4. Move the mouse again.  This triggers an `inChartArea = false` event,
   so it reuses the previous, now invalid, active elements.

This fixes chartjs#11315 under the circumstances for which I've reproduced it,
but there may be others.

src/plugins/plugin.tooltip.js

@@ -1182,7 +1182,8 @@ export class Tooltip extends Element {
 
     if (!inChartArea) {
       // Let user control the active elements outside chartArea. Eg. using Legend.
-      return lastActive;
+      // But make sure that active elements are still valid.
+      return lastActive.filter(i => this.chart.data.datasets[i.datasetIndex]);
     }
 
     // Find Active Elements for tooltips

test/specs/plugin.tooltip.tests.js

@@ -1667,6 +1667,48 @@ describe('Plugin.Tooltip', function() {
     });
   });
 
+  it('should tolerate datasets removed or added on events outside chartArea', async function() {
+    const dataset1 = {
+      label: 'Dataset 1',
+      data: [10, 20, 30],
+    };
+    const dataset2 = {
+      label: 'Dataset 2',
+      data: [10, 25, 35],
+    };
+    const chart = window.acquireChart({
+      type: 'line',
+      data: {
+        datasets: [dataset1, dataset2],
+        labels: ['Point 1', 'Point 2', 'Point 3']
+      },
+      options: {
+        plugins: {
+          tooltip: {
+            mode: 'index',
+            intersect: false
+          }
+        }
+      }
+    });
+
+    var meta = chart.getDatasetMeta(0);
+    var point = meta.data[1];
+    var expectedPoints = [jasmine.objectContaining({datasetIndex: 0, index: 1}), jasmine.objectContaining({datasetIndex: 1, index: 1})];
+
+    await jasmine.triggerMouseEvent(chart, 'mousemove', point);
+    await jasmine.triggerMouseEvent(chart, 'mousemove', {x: chart.chartArea.left - 5, y: point.y});
+
+    expect(chart.tooltip.getActiveElements()).toEqual(expectedPoints);
+
+    chart.data.datasets = [dataset1];
+    chart.update();
+
+    await jasmine.triggerMouseEvent(chart, 'mousemove', {x: 2, y: 1});
+
+    expect(chart.tooltip.getActiveElements()).toEqual([expectedPoints[0]]);
+  });
+
   describe('events', function() {
     it('should not be called on events not in plugin events array', async function() {
       var chart = window.acquireChart({