Update to Drupal 8.0.6. For more information, see https://www.drupal.org/drupal-8.0.6-release-notes

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

@@ -253,7 +253,7 @@
       // the "image" command's CKEditor dialog with a Drupal-native dialog.
       editor.addCommand('editdrupalimage', {
         allowedContent: 'img[alt,!src,width,height,!data-entity-type,!data-entity-uuid]',
-        requiredContent: 'img[alt,src,width,height,data-entity-type,data-entity-uuid]',
+        requiredContent: 'img[alt,src,data-entity-type,data-entity-uuid]',
         modes: {wysiwyg: 1},
         canUndo: true,
         exec: function (editor, data) {

core/modules/ckeditor/src/CKEditorPluginConfigurableInterface.php

@@ -42,8 +42,8 @@ interface CKEditorPluginConfigurableInterface extends CKEditorPluginInterface {
    * @param \Drupal\editor\Entity\Editor $editor
    *   A configured text editor object.
    *
-   * @return array|FALSE
-   *   A render array for the settings form, or FALSE if there is none.
+   * @return array
+   *   A render array for the settings form.
    */
   public function settingsForm(array $form, FormStateInterface $form_state, Editor $editor);