fix: fix bug

Author: sjbur

packages/devextreme/js/__internal/grids/pivot_grid/data_source/m_data_source.ts

@@ -1406,7 +1406,9 @@ class PivotGridDataSource {
       delayedLoadTask.abort();
     }
 
-    that._store?.dispose();
+    if (that._store) {
+      that._store.dispose();
+    }
 
     this._isDisposed = true;
   }

packages/devextreme/js/__internal/grids/pivot_grid/local_store/m_local_store.ts

@@ -586,7 +586,9 @@ const LocalStore = Class.inherit((function () {
     },
 
     dispose() {
-      this._dataSource?.dispose();
+      if (this._dataSource) {
+        this._dataSource.dispose();
+      }
     },
   };
 })()).include(storeDrillDownMixin);

packages/devextreme/js/__internal/grids/pivot_grid/remote_store/m_remote_store.ts

@@ -664,7 +664,9 @@ class RemoteStore {
   }
 
   dispose() {
-    this._dataSource?.dispose();
+    if (this._dataSource) {
+      this._dataSource.dispose();
+    }
   }
 }