Draft: Resolve #2855521 "Drupallink called by drupalimage"

Closes #2855521

core/modules/ckeditor/js/plugins/drupalimage/plugin.es6.js

@@ -387,12 +387,12 @@
 
   // Override image2's integration with the official CKEditor link plugin:
   // integrate with the drupallink plugin instead.
-  CKEDITOR.plugins.image2.getLinkAttributesParser = function() {
-    return CKEDITOR.plugins.drupallink.parseLinkAttributes;
-  };
-  CKEDITOR.plugins.image2.getLinkAttributesGetter = function() {
-    return CKEDITOR.plugins.drupallink.getLinkAttributes;
-  };
+  if (CKEDITOR.plugins.externals.drupallink) {
+    CKEDITOR.plugins.image2.getLinkAttributesParser = () =>
+      CKEDITOR.plugins.drupallink.parseLinkAttributes;
+    CKEDITOR.plugins.image2.getLinkAttributesGetter = () =>
+      CKEDITOR.plugins.drupallink.getLinkAttributes;
+  }
 
   // Expose an API for other plugins to interact with drupalimage widgets.
   CKEDITOR.plugins.drupalimage = {