Fix: #198 - Error thrown in ngOnDestroy if plot not yet initialized

This can occur in unit tests if you do not await the component being
stable, so this resolves #198. However, this could also happen if
ngOnDestroy is called very quickly after ngOnInit.

Author: paulirwin

projects/plotly/src/lib/plotly.component.spec.ts

@@ -242,4 +242,9 @@ describe('PlotlyComponent', () => {
         expect(component.loadTheme).not.toHaveBeenCalled();
         expect(component.themeLoader.load).not.toHaveBeenCalledOnceWith('plotly_dark');
     });
+
+    it('should not cause errors if ngOnDestroy is called before plotly is initialized', () => {
+        // note that this test intentionally does not call ngOnInit/whenStable
+        expect(() => component.ngOnDestroy()).not.toThrow();
+    });
 });

projects/plotly/src/lib/plotly.component.ts

@@ -143,9 +143,11 @@ export class PlotlyComponent implements OnInit, OnChanges, OnDestroy, DoCheck {
             this.resizeHandler = undefined;
         }
 
-        const figure = this.createFigure();
-        this.purge.emit(figure);
-        PlotlyService.remove(this.plotlyInstance!);
+        if (this.plotlyInstance) {
+            const figure = this.createFigure();
+            this.purge.emit(figure);
+            PlotlyService.remove(this.plotlyInstance);
+        }
     }
 
     ngOnChanges(changes: SimpleChanges): void {