*Steps to reproduce:*

1. Open UI Builder. Add a style sheet to the document.

2. Type in the "Add new selector field", clear the field so its empty and press return

3. Exception is thrown

 
{noformat}
IndexOutOfRangeException: Index was outside the bounds of the array.
Unity.UI.Builder.BuilderStyleSheets.CreateNewSelector (UnityEngine.UIElements.StyleSheet styleSheet) (at <1e62fa9ecc5949b1888e6e526b2b5fb3>:0)
Unity.UI.Builder.BuilderStyleSheets.OnEnter (UnityEngine.UIElements.KeyDownEvent evt) (at <1e62fa9ecc5949b1888e6e526b2b5fb3>:0)
UnityEngine.UIElements.EventCallbackFunctor`1[TEventType].Invoke (UnityEngine.UIElements.EventBase evt) (at <946e1335742a4b0ea262e6c117357e7c>:0)
UnityEngine.UIElements.EventCallbackRegistry+DynamicCallbackList.Invoke (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.BaseVisualElementPanel panel, UnityEngine.UIElements.VisualElement target) (at <946e1335742a4b0ea262e6c117357e7c>:0)
UnityEngine.UIElements.EventDispatchUtilities.HandleEvent_TrickleDownCallbacks (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.BaseVisualElementPanel panel, UnityEngine.UIElements.VisualElement element) (at <946e1335742a4b0ea262e6c117357e7c>:0)
UnityEngine.UIElements.EventDispatchUtilities.HandleEventAcrossPropagationPath (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.BaseVisualElementPanel panel, UnityEngine.UIElements.VisualElement target, System.Boolean isCapturingTarget) (at <946e1335742a4b0ea262e6c117357e7c>:0)
UnityEngine.UIElements.EventDispatchUtilities.PropagateEvent (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.BaseVisualElementPanel panel, UnityEngine.UIElements.VisualElement target, System.Boolean isCapturingTarget) (at <946e1335742a4b0ea262e6c117357e7c>:0)
UnityEngine.UIElements.EventDispatchUtilities.DispatchToFocusedElementOrPanelRoot (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.BaseVisualElementPanel panel) (at <946e1335742a4b0ea262e6c117357e7c>:0)
UnityEngine.UIElements.KeyboardEventBase`1[T].Dispatch (UnityEngine.UIElements.BaseVisualElementPanel panel) (at <946e1335742a4b0ea262e6c117357e7c>:0)
UnityEngine.UIElements.EventDispatcher.ProcessEvent (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.BaseVisualElementPanel panel) (at <946e1335742a4b0ea262e6c117357e7c>:0)
UnityEngine.UIElements.EventDispatcher.Dispatch (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.BaseVisualElementPanel panel, UnityEngine.UIElements.DispatchMode dispatchMode) (at <946e1335742a4b0ea262e6c117357e7c>:0)
UnityEngine.UIElements.BaseVisualElementPanel.SendEvent (UnityEngine.UIElements.EventBase e, UnityEngine.UIElements.DispatchMode dispatchMode) (at <946e1335742a4b0ea262e6c117357e7c>:0)
UnityEngine.UIElements.UIElementsUtility.DoDispatch (UnityEngine.UIElements.BaseVisualElementPanel panel) (at <946e1335742a4b0ea262e6c117357e7c>:0)
UnityEngine.UIElements.UIElementsUtility.UnityEngine.UIElements.IUIElementsUtility.ProcessEvent (System.Int32 instanceID, System.IntPtr nativeEventPtr, System.Boolean& eventHandled) (at <946e1335742a4b0ea262e6c117357e7c>:0)
UnityEngine.UIElements.UIEventRegistration.ProcessEvent (System.Int32 instanceID, System.IntPtr nativeEventPtr) (at <946e1335742a4b0ea262e6c117357e7c>:0)
UnityEngine.UIElements.UIEventRegistration+<>c.<.cctor>b__1_2 (System.Int32 i, System.IntPtr ptr) (at <946e1335742a4b0ea262e6c117357e7c>:0)
UnityEngine.GUIUtility.ProcessEvent (System.Int32 instanceID, System.IntPtr nativeEventPtr, System.Boolean& result) (at <4e8fd10292474a8fa2f0a5c88b71bcea>:0)
{noformat}
*Actual results:* No exception

*Expected results:* Exception is thrown

*Reproducible with versions:* 6000.0b1

*Not reproducible with versions:* 

*Can’t test with versions:* 

*Tested on (OS):* Windows 10

*Notes:*
*