Make metadata access safer at runtime

The `!` operator is a type assertion that the attribute is not null, but
does nothing at runtime to ensure this is the case. I haven't yet found
reproduction steps for this failure, but I encountered a case where
accessing `model!.sharedModel` threw an error. This commit adds optional
chaining operators, which do provide protection at runtime.

Author: shreve

src/formatter.ts

@@ -98,7 +98,7 @@ export class JupyterlabNotebookCodeFormatter extends JupyterlabCodeFormatter {
     }
 
     const metadata =
-      this.notebookTracker.currentWidget.content.model!.sharedModel.metadata;
+      this.notebookTracker.currentWidget.content.model?.sharedModel?.metadata;
 
     if (!metadata) {
       return null;