Change cell.disposed to cell.isDisposed

src/handlers/cell.ts

@@ -73,7 +73,7 @@ export class CellManager implements IDisposable {
 
   // TODO: call when the debugger stops
   private cleanupHighlight() {
-    if (!this.activeCell || this.activeCell.disposed) {
+    if (!this.activeCell || this.activeCell.isDisposed) {
       return;
     }
     const editor = this.activeCell.editor as CodeMirrorEditor;
@@ -169,7 +169,7 @@ export class CellManager implements IDisposable {
   }
 
   protected removeListener(cell: CodeCell) {
-    if (cell.disposed) {
+    if (cell.isDisposed) {
       return;
     }
     const editor = cell.editor as CodeMirrorEditor;