Merge pull request #193 from blink1073/fix-md-cell

Fix markdown mime type

src/notebook/cells/widget.ts

@@ -227,9 +227,9 @@ class MarkdownCellWidget extends BaseCellWidget {
       if (this._dirty) {
         let text = model.input.textEditor.text || DEFAULT_MARKDOWN_TEXT;
         text = sanitize(text);
-        let bundle: MimeBundle = { 'application/vnd.jupyter.markdown': text };
+        let bundle: MimeBundle = { 'text/markdown': text };
         this._rendered.dispose();
-        this._rendered = this._rendermime.render(bundle);
+        this._rendered = this._rendermime.render(bundle) || new Widget();
         this._rendered.addClass(RENDERER_CLASS);
         (this.layout as PanelLayout).addChild(this._rendered);
       }