Fix error when closing MRVA webview during extension activation

This fixes the "Webview is disposed" error which occurs when the user
closes the variant analysis webview while the extension is still
activating. We will now check whether the webview is disposed before
restoring the view.

Author: koesie10

extensions/ql-vscode/src/variant-analysis/variant-analysis-view-serializer.ts

@@ -37,6 +37,14 @@ export class VariantAnalysisViewSerializer implements WebviewPanelSerializer {
       return;
     }
 
+    // Between the time the webview is deserialized and the time the extension
+    // is fully activated, the user may close the webview. In this case, we
+    // should not attempt to restore the view.
+    let disposed = false;
+    const unregisterOnDidDispose = webviewPanel.onDidDispose(() => {
+      disposed = true;
+    });
+
     const variantAnalysisState: VariantAnalysisState =
       state as VariantAnalysisState;
 
@@ -46,18 +54,25 @@ export class VariantAnalysisViewSerializer implements WebviewPanelSerializer {
       variantAnalysisState.variantAnalysisId,
     );
     if (existingView) {
+      unregisterOnDidDispose.dispose();
       await existingView.openView();
       webviewPanel.dispose();
       return;
     }
 
+    if (disposed) {
+      return;
+    }
+
     const view = new VariantAnalysisView(
       this.ctx,
       this.app,
       variantAnalysisState.variantAnalysisId,
       manager,
     );
     await view.restoreView(webviewPanel);
+
+    unregisterOnDidDispose.dispose();
   }
 
   private waitForExtensionFullyLoaded(): Promise<