appIndicator: Do not try to initialize an indicator not in stage yet

Even if an indicator is ready we should not try to update it if it's not in
stage because we depend on the theme node

appIndicator.js

@@ -980,7 +980,17 @@ class AppIndicatorsIconActor extends St.Icon {
 
         if (indicator.isReady) {
             this._updateCustomIcons();
-            this._invalidateIcon();
+
+            if (this.get_stage()) {
+                this._invalidateIcon();
+            } else {
+                const id = this.connect('parent-set', () => {
+                    if (this.get_stage()) {
+                        this.disconnect(id);
+                        this._invalidateIcon();
+                    }
+                });
+            }
         }
 
         this.connect('destroy', () => {