A corner case check in `Instance::handleMemoryPressure`

Summary:
At some early stages of Instance initialization, it does not have a `nativeToJsBridge_`. At the same time, `handleMemoryPressure` can be called at any point in time, so we should check the pointer for not being null before calling on it.

Changelog: [Internal] Fabric-specific internal change.

Reviewed By: JoshuaGross, mdvacca

Differential Revision: D21798522

fbshipit-source-id: 6384da88784cceb493cf9810408cbb47777d3f4b

ReactCommon/cxxreact/Instance.cpp

@@ -222,7 +222,11 @@ ModuleRegistry &Instance::getModuleRegistry() {
 }
 
 void Instance::handleMemoryPressure(int pressureLevel) {
-  nativeToJsBridge_->handleMemoryPressure(pressureLevel);
+  if (nativeToJsBridge_) {
+    // This class resets `nativeToJsBridge_` only in the destructor,
+    // hence a race is not possible there.
+    nativeToJsBridge_->handleMemoryPressure(pressureLevel);
+  }
 }
 
 std::shared_ptr<CallInvoker> Instance::getJSCallInvoker() {