Fix missed case in "observe" type check (#2919)

* Make sure that assumed TypeError would be thrown

* Check that ThrowError would be thrown in case of null observer object

* Add null check for subscribe method

Author: zerobias

src/createStore.js

@@ -234,7 +234,7 @@ export default function createStore(reducer, preloadedState, enhancer) {
        * emission of values from the observable.
        */
       subscribe(observer) {
-        if (typeof observer !== 'object') {
+        if (typeof observer !== 'object' || observer === null) {
           throw new TypeError('Expected the observer to be an object.')
         }
 

test/createStore.spec.js

@@ -629,11 +629,15 @@ describe('createStore', () => {
 
         expect(function() {
           obs.subscribe()
-        }).toThrow()
+        }).toThrowError(new TypeError('Expected the observer to be an object.'))
+
+        expect(function() {
+          obs.subscribe(null)
+        }).toThrowError(new TypeError('Expected the observer to be an object.'))
 
         expect(function() {
           obs.subscribe(() => {})
-        }).toThrow()
+        }).toThrowError(new TypeError('Expected the observer to be an object.'))
 
         expect(function() {
           obs.subscribe({})