Fix run button disable. Switching to tutorial and back to samples (#1849) (#2206)

Hi, This should fix the problem #1849 
I think that the first time the model is created and then the get is
done. For `samples` kataSection is `undefined` and I don't know why the
EntryPoint is missing. What do you think? Probably it's only a
workaround.

Author: thecoder93

playground/src/editor.tsx

@@ -299,6 +299,7 @@ export function Editor(props: {
     return () => {
       log.info("Disposing a monaco editor");
       window.removeEventListener("resize", onResize);
+      props.languageService.closeDocument(srcModel.uri.toString());
       newEditor.dispose();
     };
   }, []);