Fix nulls in undo.ts

src/cursor-doc/undo.ts

@@ -114,17 +114,21 @@ export class UndoManager<T> {
   undo(c: T) {
     if (this.undos.length) {
       const step = this.undos.pop();
-      step.undo(c);
-      this.redos.push(step);
+      if (step) {
+        step.undo(c);
+        this.redos.push(step);
+      }
     }
   }
 
   /** Performs the top redo operation on the document (if it exists), moving it back onto the undo stack. */
   redo(c: T) {
     if (this.redos.length) {
       const step = this.redos.pop();
-      step.redo(c);
-      this.undos.push(step);
+      if (step) {
+        step.redo(c);
+        this.undos.push(step);
+      }
     }
   }
 }