Make getPublicInstance type safe

Author: iamdustan

src/renderers/shared/fiber/ReactFiberCommitWork.js

@@ -463,8 +463,14 @@ module.exports = function<T, P, I, TI, PI, C, CX>(
     }
     const ref = finishedWork.ref;
     if (ref) {
-      const instance = getPublicInstance(finishedWork.stateNode);
-      ref(instance);
+      const instance = finishedWork.stateNode;
+      switch (finishedWork.tag) {
+        case HostComponent:
+          ref(getPublicInstance(instance));
+          break;
+        default:
+          ref(instance);
+      }
     }
   }