strict null checks in fileeditor

packages/fileeditor/src/widget.ts

@@ -2,7 +2,7 @@
 // Distributed under the terms of the Modified BSD License.
 
 import {
-  IChangedArgs
+  IChangedArgs, PathExt
 } from '@jupyterlab/coreutils';
 
 import {
@@ -56,8 +56,13 @@ class FileEditor extends CodeEditorWrapper implements DocumentRegistry.IReadyWid
     if (context.model.modelDB.isCollaborative) {
       let modelDB = context.model.modelDB;
       modelDB.connected.then(() => {
+        let collaborators = modelDB.collaborators;
+        if (!collaborators) {
+          return;
+        }
+
         // Setup the selection style for collaborators
-        let localCollaborator = modelDB.collaborators.localCollaborator;
+        let localCollaborator = collaborators.localCollaborator;
         this.editor.uuid = localCollaborator.sessionId;
 
         this.editor.selectionStyle = {
@@ -65,7 +70,7 @@ class FileEditor extends CodeEditorWrapper implements DocumentRegistry.IReadyWid
           color: localCollaborator.color
         };
 
-        modelDB.collaborators.changed.connect(this._onCollaboratorsChanged, this);
+        collaborators.changed.connect(this._onCollaboratorsChanged, this);
         // Trigger an initial onCollaboratorsChanged event.
         this._onCollaboratorsChanged();
       });
@@ -153,7 +158,7 @@ class FileEditor extends CodeEditorWrapper implements DocumentRegistry.IReadyWid
     const path = this._context.path;
 
     editor.model.mimeType = this._mimeTypeService.getMimeTypeByFilePath(path);
-    this.title.label = path.split(':').pop().split('/').pop();
+    this.title.label = PathExt.basename(path.split(':').pop()!);
   }
 
   /**
@@ -163,8 +168,12 @@ class FileEditor extends CodeEditorWrapper implements DocumentRegistry.IReadyWid
   private _onCollaboratorsChanged(): void {
     // If there are selections corresponding to non-collaborators,
     // they are stale and should be removed.
+    let collaborators = this._context.model.modelDB.collaborators;
+    if (!collaborators) {
+      return;
+    }
     for (let key of this.editor.model.selections.keys()) {
-      if (!this._context.model.modelDB.collaborators.has(key)) {
+      if (!collaborators.has(key)) {
         this.editor.model.selections.delete(key);
       }
     }