Fix for classic editor (#3013)

* Check for valid return value from wp.data.dispatch.

Author: brianhogg

.changelogs/fix_classic-editor.yml

@@ -0,0 +1,6 @@
+significance: patch
+type: fixed
+links:
+  - "#3012"
+entry: Additional check to avoid conflict with certain plugins alongside the
+  Classic Editor.

assets/js/private/llms-metaboxes.js

@@ -460,6 +460,7 @@
 			const canEditPost =
 				wp?.data &&
 				typeof wp.data.dispatch === 'function' &&
+				wp.data.dispatch( 'core/editor' ) !== null &&
 				typeof wp.data.dispatch( 'core/editor' ).editPost === 'function';
 
 			if ( ! canEditPost ) {