Fix soft crash when editing message

Fixes https://github.com/matrix-org/riot-web-rageshakes/issues/2504

src/components/views/rooms/BasicMessageComposer.js

@@ -94,7 +94,8 @@ export default class BasicMessageEditor extends React.Component {
         this._emoticonSettingHandle = null;
     }
 
-    componentDidUpdate(prevProps) {
+    // TODO: [REACT-WARNING] Move into better lifecycle position
+    UNSAFE_componentWillUpdate(prevProps) {
         if (this.props.placeholder !== prevProps.placeholder && this.props.placeholder) {
             const {isEmpty} = this.props.model;
             if (isEmpty) {