Fix-broken-ref (#15846)

An example of failed smoke tests run prior to this fix: https://app.circleci.com/pipelines/github/metabase/metabase/15496/workflows/d4937774-3ef6-412b-a7c8-a6b9e47b5515/jobs/594197

frontend/src/metabase/admin/datamodel/containers/FieldApp.jsx

@@ -90,8 +90,6 @@ export default class FieldApp extends React.Component {
     tab: "general",
   };
 
-  saveStatus: null;
-
   props: {
     databaseId: DatabaseId,
     tableId: TableId,
@@ -115,6 +113,11 @@ export default class FieldApp extends React.Component {
     params: any,
   };
 
+  constructor(props) {
+    super(props);
+    this.saveStatusRef = React.createRef();
+  }
+
   async UNSAFE_componentWillMount() {
     const {
       databaseId,
@@ -147,9 +150,9 @@ export default class FieldApp extends React.Component {
   }
 
   linkWithSaveStatus = (saveMethod: Function) => async (...args: any[]) => {
-    this.saveStatus && this.saveStatus.setSaving();
+    this.saveStatusRef.current && this.saveStatusRef.current.setSaving();
     await saveMethod(...args);
-    this.saveStatus && this.saveStatus.setSaved();
+    this.saveStatusRef.current && this.saveStatusRef.current.setSaved();
   };
 
   onUpdateFieldProperties = this.linkWithSaveStatus(async fieldProps => {
@@ -238,7 +241,7 @@ export default class FieldApp extends React.Component {
                 </div>
               )}
               <div className="absolute top right mt4 mr4">
-                <SaveStatus ref={this.saveStatus} />
+                <SaveStatus ref={this.saveStatusRef} />
               </div>
 
               {section == null || section === "general" ? (