Fix check for Prosemirror editor

Fixes #35

Author: annda

script.js

@@ -82,7 +82,7 @@
      * @param {string} id The newly uploaded file ID
      */
     function insertSyntax(id) {
-        if (proseMirrorIsActive) {
+        if (typeof window.proseMirrorIsActive !== 'undefined' && window.proseMirrorIsActive === true) {
             const pm = window.Prosemirror.view;
             const imageNode = pm.state.schema.nodes.image.create({id: id});
             pm.dispatch(pm.state.tr.replaceSelectionWith(imageNode));