Fix Composite set not accepting null parameter

Change-Id: Ib0e2ba6402988ed5e6472a1d36c20555ee6bac05

Author: tbuschto

src/tabris/widgets/Composite.js

@@ -56,7 +56,7 @@ export default class Composite extends Widget {
   }
 
   set(props) {
-    if (('children' in props) && !(props.children instanceof Function)) {
+    if (props && ('children' in props) && !(props.children instanceof Function)) {
       throw new Error('You may not override children with a non-function');
     }
     return super.set(props);

test/tabris/widgets/Widget.test.js

@@ -68,6 +68,10 @@ describe('Widget', function() {
       expect(widget).to.be.instanceof(Widget);
     });
 
+    it('accepts set(null)', function() {
+      expect(() => widget.set(null)).not.to.throw(Error);
+    });
+
     it('translates background color to a color shader', function() {
       widget.set({background: 'rgba(1, 2, 3, 0.5)'});