fix #7869 -- jupyter/course/nbgrader: minor bug showing warning about protected cell

- root cause was mangling of an optimization when checking changing
  input of a cell

Author: williamstein

src/packages/jupyter/redux/actions.ts

@@ -385,7 +385,7 @@ export abstract class JupyterActions extends Actions<JupyterStoreState> {
   // Might throw a CellWriteProtectedException
   public set_cell_input(id: string, input: string, save = true): void {
     if (!this.store) return;
-    if (this.store.getIn(["this.st", id, "input"]) == input) {
+    if (this.store.getIn(["cells", id, "input"]) == input) {
       // nothing changed.   Note, I tested doing the above check using
       // both this.syncdb and this.store, and this.store is orders of magnitude faster.
       return;