popStack method Null safety

void popStack()

Implementation

void popStack() {
  void f(final List<Value>? st) {
    if (st != null) {
      st[0] = st[1];
      st[1] = st[2];
      st[2] = st[3];
    }
  }

  f(_stack);
  f(_imaginaryStack);
  display.window = 0;
  needsSave = true;
  _clxDone = false;
}