Fix case when property 'observer' in modificationObserver is not defined (#866)

docs/CHANGELOG.md

@@ -6,6 +6,7 @@
 - `Refactoring` — Created universal Flipper class that responses for navigation by keyboard inside of any Toolbars
 - `Fix` — First CMD+A on block with now uses default behaviour. Fixed problem with second CMD+A after selection clearing [#827](https://github.com/codex-team/editor.js/issues/827)
 - `Improvements` — Style of inline selection and selected blocks improved
+- `Fix` - Fixed problem when property 'observer' in modificationObserver is not defined
 
 ### 2.15
 

src/components/modules/modificationsObserver.ts

@@ -47,7 +47,9 @@ export default class ModificationsObserver extends Module {
    */
   public destroy() {
     this.mutationDebouncer = null;
-    this.observer.disconnect();
+    if (this.observer) {
+      this.observer.disconnect();
+    }
     this.observer = null;
     this.nativeInputs.forEach((input) => this.Editor.Listeners.off(input, 'input', this.mutationDebouncer));
   }