[fixed] Modal error when backdrop is false

fixes react-bootstrap#1074

src/Modal.js

@@ -226,6 +226,16 @@ const Modal = React.createClass({
   },
 
   _setDialogRef(ref){
+    // issue #1074
+    // due to: https://github.com/facebook/react/blob/v0.13.3/src/core/ReactCompositeComponent.js#L842
+    //
+    // when backdrop is `false` react hasn't had a chance to reassign the refs to a usable object, b/c there are no other
+    // "classic" refs on the component (or they haven't been processed yet)
+    // TODO: Remove the need for this in next breaking release
+    if (Object.isFrozen(this.refs) && !Object.keys(this.refs).length) {
+      this.refs = {};
+    }
+
     this.refs.dialog = ref;
 
     //maintains backwards compat with older component breakdown

test/ModalSpec.js

@@ -160,6 +160,17 @@ describe('Modal', function () {
     assert.match(dialog.props.className, /\btestCss\b/);
   });
 
+  it('Should assign refs correctly when no backdrop', function () {
+
+    let test = () => render(
+      <Modal show backdrop={false} onHide={function () {}}>
+        <strong>Message</strong>
+      </Modal>
+    , mountPoint);
+
+    expect(test).not.to.throw();
+  });
+
   it('Should pass transition callbacks to Transition', function (done) {
     let count = 0;
     let increment = ()=> count++;