Don't format if the notebook type is undetermined

In the event that `getNotebookType()` is null, an error is shown from
within `getFormattersToUse()` because it can't determine the list of
default formatters. This makes sense when using an unconfigured
language, but in the event of null, this seems to mean the notebook
isn't fully initialized and formatting shouldn't even be attempted.

src/formatter.ts

@@ -238,6 +238,10 @@ export class JupyterlabNotebookCodeFormatter extends JupyterlabCodeFormatter {
     }
     try {
       this.working = true;
+      if (this.getNotebookType() === null) {
+        this.working = false;
+        return;
+      }
       const selectedCells = this.getCodeCells(selectedOnly, notebook);
       if (selectedCells.length === 0) {
         this.working = false;