workspacesView: Check for primaryView being null

This is possible according to the getter, and triggers warnings
seen when going back from overview via gestures.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1643>

js/ui/workspacesView.js

@@ -873,7 +873,10 @@ class WorkspacesDisplay extends St.Widget {
     }
 
     activeWorkspaceHasMaximizedWindows() {
-        return this._getPrimaryView().getActiveWorkspace().hasMaximizedWindows();
+        const primaryView = this._getPrimaryView();
+        return primaryView
+            ? primaryView.getActiveWorkspace().hasMaximizedWindows()
+            : false;
     }
 
     _onRestacked(overview, stackIndices) {