Updates to completer strict nulls for extension

packages/completer/src/index.ts

@@ -61,7 +61,7 @@ namespace ICompletionManager {
     /**
      * The host editor for the completer.
      */
-    editor: CodeEditor.IEditor;
+    editor: CodeEditor.IEditor | null;
 
     /**
      * The session used by the completer to make API requests.

packages/completer/src/widget.ts

@@ -494,7 +494,7 @@ namespace Completer {
     /**
      * The semantic parent of the completer widget, its referent editor.
      */
-    editor?: CodeEditor.IEditor;
+    editor?: CodeEditor.IEditor | null;
 
     /**
      * The model for the completer widget.