Prevent undefined errors on dashboard view

Changeset c2b74d2ff7ed does not work for dashboard view, as it has no related
screen and it does not have the same tab options as form tabs
issue6867
review36721002

src/tab.js

@@ -61,7 +61,8 @@
                     icon: 'glyphicon-time',
                     label: Sao.i18n.gettext('View Logs...'),
                 }, {
-                    id: Sao.common.MODELHISTORY.contains(this.screen.model_name) ?
+                    id: (this.screen &&
+                        Sao.common.MODELHISTORY.contains(this.screen.model_name)) ?
                         'revision': null,
                     icon: 'glyphicon-time',
                     label: Sao.i18n.gettext('Show revisions...'),
@@ -197,7 +198,7 @@
                     group = null;
                     return;
                 }
-                if (!item.id) {
+                if (!item.id || !this[item.id]) {
                     return;
                 }
                 if (!group) {