fix(input): corrected NPE when input goes away

Closes #392

lib/directive/input_select.dart

@@ -62,7 +62,7 @@ class InputSelectDirective implements NgAttachAware {
   }
 
   /**
-   * This method invalidates the current state of the selector and forces a rerendering of the
+   * This method invalidates the current state of the selector and forces a re-rendering of the
    * options using the [Scope.$evalAsync].
    */
   dirty() {
@@ -175,7 +175,13 @@ class _SingleSelectMode extends _SelectMode {
     var found = false;
     _forEachOption((option, i) {
       if (option == _unknownOption) return;
-      var selected = value == null ? option == _nullOption : expando[option].ngValue == value;
+      var selected;
+      if (value == null) {
+        selected = option == _nullOption;
+      } else {
+        OptionValueDirective optionValueDirective = expando[option];
+        selected = optionValueDirective == null ? false : optionValueDirective.ngValue == value;
+      }
       found = found || selected;
       option.selected = selected;
     });

test/directive/input_select_spec.dart

@@ -376,6 +376,28 @@ main() {
           });
         });
       });
+
+      it('issue #392', () {
+        _.compile(
+            '<div>' +
+              '<div ng-if="attached">' +
+                '<select ng-model="model">' +
+                  '<option value="a">foo</option>' +
+                  '<option value="b">bar</option>' +
+                '</select>' +
+              '</div>' +
+            '</div>');
+        _.rootScope.model = 'a';
+        _.rootScope.attached = true;
+        _.rootScope.$apply();
+        expect(_.rootElement).toEqualSelect([['a'], 'b']);
+        _.rootScope.attached = false;
+        _.rootScope.$apply();
+        expect(_.rootElement).toEqualSelect([]);
+        _.rootScope.attached = true;
+        _.rootScope.$apply();
+        expect(_.rootElement).toEqualSelect([['a'], 'b']);
+      });
     });