Disabled join button if no user is selected

js/index.js

@@ -23,6 +23,8 @@
                 for (var i = 0; i < users.length; i++) {
                     $userSelector.append('<option value="' + i + '">' + users[i] + '</option>');
                 }
+                
+                updateJoinButtonStatus();
             });
         };
     }
@@ -71,6 +73,14 @@
         $('.primary-identity-icon').tooltip();
     }
 
+    /**
+     * Enable or disable the join button if there are selected users or not.
+     */
+    function updateJoinButtonStatus() {
+        var selectedUserIndex = $('#user-selector-group select').val();
+        $('#send-join-request-btn').prop("disabled", selectedUserIndex === null);
+    }
+
     // When the document is loaded
     $(document).ready(function () {
 
@@ -93,6 +103,8 @@
             $('#info-message-alert').addClass('hide');
         });
 
+        $('#search-user-modal').on('shown.bs.modal', updateJoinButtonStatus);
+
         loadTooltips();
     });