Fabric: Added assert in ShadowNode

Summary:
It's not allowed to return nullptr from the callback. The assert ensures it which is helpful during development.

Probably, we should consider using `gsl::not_null<>` here.

Changelog: [Internal] Fabric-specific internal change.

Reviewed By: sammy-SC

Differential Revision: D21149891

fbshipit-source-id: a5f77b35029f22b499491721036405682f812a38

ReactCommon/fabric/core/shadownode/ShadowNode.cpp

@@ -263,6 +263,10 @@ ShadowNode::Unshared ShadowNode::cloneTree(
 
   auto newShadowNode = callback(*oldShadowNode);
 
+  assert(
+      newShadowNode &&
+      "`callback` returned `nullptr` which is not allowed value.");
+
   auto childNode = newShadowNode;
 
   for (auto it = ancestors.rbegin(); it != ancestors.rend(); ++it) {