Check that notification target is not null (#3449)

* Check that target is not null

* Prevent display of "negative" notification coutner

* spelling fix

InvenTree/templates/js/translated/notification.js

@@ -174,6 +174,12 @@ function updateNotificationReadState(btn, panel_caller=false) {
             } else {
                 count = count + 1;
             }
+
+            // Prevent negative notification count
+            if (count < 0) {
+                count = 0;
+            }
+
             // update notification indicator now
             updateNotificationIndicator(count);