EventTarget (native)/InstanceHandle (JS) is not optional, assert that it's non-null

Summary:
When EventTarget is created in createNode, it's currently treated as optional in native, but it's not optional in JS. We should assert that it's non-null to make the contract more clear.

For now, we keep it as an assert so we catch issues in debug/dev, but it won't impact prod.

Changelog: [Internal]

Reviewed By: mdvacca

Differential Revision: D27002506

fbshipit-source-id: c541dc02d3de22df19292deaef1a3b68fe57ba1f

ReactCommon/react/renderer/uimanager/primitives.h

@@ -10,6 +10,7 @@
 #include <folly/dynamic.h>
 #include <jsi/JSIDynamic.h>
 #include <jsi/jsi.h>
+#include <react/debug/react_native_assert.h>
 #include <react/renderer/core/EventHandler.h>
 #include <react/renderer/core/ShadowNode.h>
 
@@ -110,6 +111,7 @@ inline static SharedEventTarget eventTargetFromValue(
     jsi::Runtime &runtime,
     jsi::Value const &eventTargetValue,
     jsi::Value const &tagValue) {
+  react_native_assert(!eventTargetValue.isNull());
   if (eventTargetValue.isNull()) {
     return nullptr;
   }