fix(tree-grid): added check for grid (#9212)

Co-authored-by: Galina Edinakova <[email protected]>

Author: igdmdimitrov

projects/igniteui-angular/src/lib/grids/tree-grid/tree-grid-api.service.ts

@@ -12,8 +12,9 @@ export class IgxTreeGridAPIService extends GridBaseAPIService<IgxTreeGridCompone
 
     public get_all_data(transactions?: boolean): any[] {
         const grid = this.grid;
-        const data = transactions ? grid.dataWithAddedInTransactionRows : grid.flatData;
-        return data ? data : [];
+        let data = grid && grid.flatData ? grid.flatData : [];
+        data = transactions ? grid.dataWithAddedInTransactionRows : data;
+        return data;
     }
 
     public get_summary_data() {