[FIX] web: better error handling in view and action manager

Before this commit, it could happen that a crash at the wrong time (for
example, while loading a view) cause the web client to crash for various
reasons.  For example, it assumes that the active view always has a
valid controller.  Or the action manager could not detach the previous
view, so we had the 'double view' rendering in the web client.

With this commit, we add some protections at various points.  Those
safety measures should help the user keep working even if a specific
view did crash.

addons/web/static/src/js/chrome/action_manager.js

@@ -185,13 +185,17 @@ var ViewManagerAction = WidgetAction.extend({
      * @param {int} [scrollTop] the number of pixels to scroll
      */
     setScrollTop: function(scrollTop) {
-        this.widget.active_view.controller.setScrollTop(scrollTop);
+        var viewController = this.widget.active_view.controller;
+        if (viewController) {
+            viewController.setScrollTop(scrollTop);
+        }
     },
     /**
      * @return {int} the number of pixels the webclient is scrolled when leaving the action
      */
     getScrollTop: function() {
-        return this.widget.active_view.controller.getScrollTop();
+        var viewController = this.widget.active_view.controller;
+        return viewController ? viewController.getScrollTop() : 0;
     },
     /**
      * @return {Array} array of Objects that will be interpreted to display the breadcrumbs
@@ -340,6 +344,16 @@ var ActionManager = Widget.extend({
                 // Hide the main ControlPanel for widgets that do not use it
                 self.main_control_panel.do_hide();
             }
+
+            // most of the time, the self.$el element should already be empty,
+            // because we detached the old action just a few line up.  However,
+            // it may happen that it is not empty, for example when a view
+            // manager was unable to load a view because of a crash.  In any
+            // case, this is done as a safety measure to avoid the 'double view'
+            // situation that we had when the web client was unable to recover
+            // from a crash.
+            self.$el.empty();
+
             dom.append(self.$el, new_widget_fragment, {
                 in_DOM: self.is_in_DOM,
                 callbacks: [{widget: self.inner_widget}],

addons/web/static/src/js/chrome/view_manager.js

@@ -59,7 +59,8 @@ var ViewManager = Widget.extend(ControlPanelMixin, {
      */
     on_detach_callback: function() {
         this.is_in_DOM = false;
-        if (this.active_view && this.active_view.controller.on_detach_callback) {
+        var controller = this.active_view && this.active_view.controller;
+        if (controller && controller.on_detach_callback) {
             this.active_view.controller.on_detach_callback();
         }
     },
@@ -282,7 +283,7 @@ var ViewManager = Widget.extend(ControlPanelMixin, {
                         view.$fragment = view.$fragment.contents();
                         view.loaded.resolve();
                     });
-                });
+                }).fail(view.loaded.reject.bind(view.loaded));
             } else {
                 view.loaded = view.loaded.then(function() {
                     view_options = _.extend({}, view_options, self.env);