Custom expression editor: avoid null ref (#19586)

Seems that the very first focus happens when the element ref isn't ready
yet (the component did not completely get mounted).

frontend/src/metabase/query_builder/components/expressions/ExpressionEditorTextfield.jsx

@@ -231,8 +231,10 @@ export default class ExpressionEditorTextfield extends React.Component {
 
   handleFocus = () => {
     this.setState({ isFocused: true });
-    const { editor } = this.input.current;
-    this.handleCursorChange(editor.selection);
+    if (this.input.current) {
+      const { editor } = this.input.current;
+      this.handleCursorChange(editor.selection);
+    }
   };
 
   handleInputBlur = e => {