fix(refUpdates): Adds defensive check to 'unobserve' method

On updates (re-rendering) there are cases where the ref callback gets called twice: first
to nullify the DOM node and again when setting the new DOM node. ReactDOM.createPortal()
for example causes this when it's re-rendered, causing an exception when we call
unobserve() on a null node. Future updates should change the way we handle refs to a more
solid solution.

Author: Rendez

src/IntersectionObserver.js

@@ -135,7 +135,9 @@ export default class IntersectionObserver extends React.Component {
     }
 
     unobserve() {
-        IntersectionObserverContainer.unobserve(this);
+        if (this.target != null) {
+            IntersectionObserverContainer.unobserve(this);
+        }
     }
 
     reobserve() {

src/__tests__/IntersectionObserver.spec.js

@@ -236,6 +236,22 @@ describe('update', () => {
         });
         expect(instance.shouldResetObserver).toBeTruthy();
     });
+
+    test('should be defensive against unobserving nullified nodes', () => {
+        const spy = jest.spyOn(IntersectionObserverContainer, 'unobserve');
+        const component = (
+            <IntersectionObserver onChange={noop}>
+                <span />
+            </IntersectionObserver>
+        );
+        const tree = renderer.create(component, {
+            createNodeMock: () => target,
+        });
+        tree.getInstance().target = null;
+        tree.getInstance().unobserve();
+
+        expect(spy).not.toBeCalled();
+    });
 });
 
 describe('callback', () => {