insertRenderObjectChild method Null safety
- covariant RenderObject child,
- covariant IndexedSlot<Element?> slot
override
    Insert the given child into renderObject at the given slot.
The semantics of slot are determined by this element. For example, if
this element has a single child, the slot should always be null. If this
element has a list of children, the previous sibling element wrapped in an
IndexedSlot is a convenient value for the slot.
Implementation
@override
void insertRenderObjectChild(RenderObject child, IndexedSlot<Element?> slot) {
  final ContainerRenderObjectMixin<RenderObject, ContainerParentDataMixin<RenderObject>> renderObject = this.renderObject;
  assert(renderObject.debugValidateChild(child));
  renderObject.insert(child, after: slot.value?.renderObject);
  assert(renderObject == this.renderObject);
}