fix None[1] assignment bug

src/main/java/chocopy/pa3/CodeGenImpl.java

@@ -450,6 +450,8 @@ public class CodeGenImpl extends CodeGenBase {
 
                     betterBackend.emitPop(tmpReg, "restore value to assign");
 
+                    backend.emitBEQZ(listReg, errorNone, "Assign list element: Exception: NullPointerException");
+
                     emitSetListElement(listReg, indexReg, lenReg, elementReg);
                 }
                 else {