#40357 Probable fix

src/vs/workbench/api/electron-browser/mainThreadTreeViews.ts

@@ -67,7 +67,7 @@ class TreeViewDataProvider implements ITreeViewDataProvider {
 				return this.postGetElements(elements);
 			}, err => {
 				this.messageService.show(Severity.Error, err);
-				return null;
+				return [];
 			});
 	}
 
@@ -80,7 +80,7 @@ class TreeViewDataProvider implements ITreeViewDataProvider {
 				return this.postGetElements(children);
 			}, err => {
 				this.messageService.show(Severity.Error, err);
-				return null;
+				return [];
 			});
 	}
 

src/vs/workbench/api/node/extHostTreeViews.ts

@@ -114,7 +114,7 @@ class ExtHostTreeView<T> extends Disposable {
 							}
 							return null;
 						})
-				))).then(extTreeItems => extTreeItems.map((({ element, extTreeItem }) => this.createTreeItem(element, extTreeItem, parentHandle))));
+				))).then(extTreeItems => coalesce(extTreeItems).map((({ element, extTreeItem }) => this.createTreeItem(element, extTreeItem, parentHandle))));
 	}
 
 	getExtensionElement(treeItemHandle: TreeItemHandle): T {