fix(middleware): do a null check in mediator methods (#4913)

src/js/tech/middleware.js

@@ -96,7 +96,9 @@ function executeRight(mws, method, value, terminated) {
   for (let i = mws.length - 1; i >= 0; i--) {
     const mw = mws[i];
 
-    mw[method](terminated, value);
+    if (mw[method]) {
+      mw[method](terminated, value);
+    }
   }
 }
 

test/unit/tech/middleware.test.js

@@ -394,3 +394,18 @@ QUnit.test('setSource will select all middleware of a given type, until src chan
   middleware.getMiddleware('video/foo').pop();
   middleware.getMiddleware('video/foo').pop();
 });
+
+QUnit.test('a middleware without a mediator method will not throw an error', function(assert) {
+  let pauseCalled = 0;
+  const myMw = {};
+  const mwFactory = () => myMw;
+  const mwFactory2 = () => ({
+    pause() {
+      pauseCalled++;
+    }
+  });
+
+  middleware.mediate([mwFactory(), mwFactory2()], {pause: () => {}}, 'pause');
+
+  assert.equal(pauseCalled, 1, 'pauseCalled was called once and no error was thrown');
+});