Revert "Minor change type nullability (#112778)" (#113246)

This reverts commit 21fab6f9.

packages/flutter/lib/src/rendering/sliver_list.dart

@@ -239,7 +239,7 @@ class RenderSliverList extends RenderSliverMultiBoxAdaptor {
         if (child == null || indexOf(child!) != index) {
           // We are missing a child. Insert it (and lay it out) if possible.
           child = insertAndLayoutChild(childConstraints,
-            after: trailingChildWithLayout!,
+            after: trailingChildWithLayout,
             parentUsesSize: true,
           );
           if (child == null) {

packages/flutter/lib/src/rendering/sliver_multi_box_adaptor.dart

@@ -481,11 +481,12 @@ abstract class RenderSliverMultiBoxAdaptor extends RenderSliver
   @protected
   RenderBox? insertAndLayoutChild(
     BoxConstraints childConstraints, {
-    required RenderBox after,
+    required RenderBox? after,
     bool parentUsesSize = false,
   }) {
     assert(_debugAssertChildListLocked());
-    final int index = indexOf(after) + 1;
+    assert(after != null);
+    final int index = indexOf(after!) + 1;
     _createOrObtainChild(index, after: after);
     final RenderBox? child = childAfter(after);
     if (child != null && indexOf(child) == index) {