UI: Fix show toast in single dashboard mode

@@ -15,7 +15,7 @@
     limitations under the License.
 
 -->
-<div class="tb-dashboard-page mat-content" [ngClass]="{'mobile-app': isMobileApp && !isEdit}" style="padding-top: 150px;"
+<div class="tb-dashboard-page mat-content" [ngClass]="{'mobile-app': isMobileApp && !isEdit}" style="padding-top: 150px;" tb-toast toastTarget="dashboardRoot"
      fxFlex tb-fullscreen [fullscreen]="widgetEditMode || iframeMode || forceFullscreen || isFullscreen">
   <tb-hotkeys-cheatsheet #cheatSheetComponent></tb-hotkeys-cheatsheet>
   <section class="tb-dashboard-toolbar"

@@ -262,35 +262,35 @@ export class WidgetContext {
   showSuccessToast(message: string, duration: number = 1000,
                    verticalPosition: NotificationVerticalPosition = 'bottom',
                    horizontalPosition: NotificationHorizontalPosition = 'left',
-                   target?: string) {
+                   target: string = 'dashboardRoot') {
     this.showToast('success', message, duration, verticalPosition, horizontalPosition, target);
   }
 
   showInfoToast(message: string,
                 verticalPosition: NotificationVerticalPosition = 'bottom',
                 horizontalPosition: NotificationHorizontalPosition = 'left',
-                target?: string) {
+                target: string = 'dashboardRoot') {
     this.showToast('info', message, undefined, verticalPosition, horizontalPosition, target);
   }
 
   showWarnToast(message: string,
                 verticalPosition: NotificationVerticalPosition = 'bottom',
                 horizontalPosition: NotificationHorizontalPosition = 'left',
-                target?: string) {
+                target: string = 'dashboardRoot') {
     this.showToast('warn', message, undefined, verticalPosition, horizontalPosition, target);
   }
 
   showErrorToast(message: string,
                  verticalPosition: NotificationVerticalPosition = 'bottom',
                  horizontalPosition: NotificationHorizontalPosition = 'left',
-                 target?: string) {
+                 target: string = 'dashboardRoot') {
     this.showToast('error', message, undefined, verticalPosition, horizontalPosition, target);
   }
 
   showToast(type: NotificationType, message: string, duration: number,
             verticalPosition: NotificationVerticalPosition = 'bottom',
             horizontalPosition: NotificationHorizontalPosition = 'left',
-            target?: string) {
+            target: string = 'dashboardRoot') {
     this.store.dispatch(new ActionNotificationShow(
       {
         message,