bugfix: virtualScrollViewport

src/components/flatTree/flatTree.component.ts

@@ -48,6 +48,12 @@ export class FlatTreeComponent implements AfterViewChecked {
   uncollapsedLevels : Set<string> = new Set()
 
   ngAfterViewChecked(){
+    /**
+     * if useDefaultList is true, virtualscrollViewPort will be undefined
+     */
+    if (!this.virtualScrollViewPort) {
+      return
+    }
     const currentTotalDataLength = this.virtualScrollViewPort.getDataLength()
     const previousDataLength = this.totalDataLength