Make sure that the watchdog instance exists before destroying itself.

src/ckeditor.jsx

@@ -74,7 +74,7 @@ export default class CKEditor extends React.Component {
 
 	// Destroy the editor before unmouting the component.
 	componentWillUnmount() {
-		this._destroyEditor();
+		return this._destroyEditor();
 	}
 
 	// Render a <div> element which will be replaced by CKEditor.
@@ -157,10 +157,19 @@ export default class CKEditor extends React.Component {
 
 	/**
 	 * Destroys the editor by destroying the watchdog.
+	 *
+	 * @returns {Promise}
 	 */
 	_destroyEditor() {
-		this.watchdog.destroy();
-		this.watchdog = null;
+		// It may happen during the tests that the watchdog instance is not assigned before destroying itself. See: #197.
+		if ( !this.watchdog ) {
+			return Promise.resolve();
+		}
+
+		return this.watchdog.destroy()
+			.then( () => {
+				this.watchdog = null;
+			} );
 	}
 
 	/**