Add Component menu breaks if VV window is closed

[perryprog]

1. View variables a thing.
2. Server components > add component. (Doesn't happen on adding client components so probably a VV session thing.)
3. Close original view variables window.
4. Try to actually add a component with the still-open component menu.
5. Explode.

Stack trace

Unhandled exception. System.NullReferenceException: Object reference not set to an instance of an object.
   at Robust.Client.ViewVariables.ClientViewVariablesManager.RequestData(ViewVariablesRemoteSession session, ViewVariablesRequest meta) in /Users/perry/git/space-station-14/RobustToolbox/Robust.Client/ViewVariables/ClientViewVariablesManager.cs:line 338
   at Robust.Client.ViewVariables.Instances.ViewVariablesInstanceEntity.GetValidServerComponentsForAdding() in /Users/perry/git/space-station-14/RobustToolbox/Robust.Client/ViewVariables/Instances/ViewVariablesInstanceEntity.cs:line 415
   at Robust.Client.ViewVariables.Instances.ViewVariablesInstanceEntity.TryAdd(AddButtonPressedEventArgs eventArgs) in /Users/perry/git/space-station-14/RobustToolbox/Robust.Client/ViewVariables/Instances/ViewVariablesInstanceEntity.cs:line 428
   at System.Threading.Tasks.Task.<>c.<ThrowAsync>b__128_0(Object state)
   at Robust.Shared.Asynchronous.RobustSynchronizationContext.ProcessPendingTasks() in /Users/perry/git/space-station-14/RobustToolbox/Robust.Shared/Asynchronous/RobustSynchronizationContext.cs:line 60
   at Robust.Shared.Asynchronous.TaskManager.ProcessPendingTasks() in /Users/perry/git/space-station-14/RobustToolbox/Robust.Shared/Asynchronous/TaskManager.cs:line 28
   at Robust.Client.GameController.Input(FrameEventArgs frameEventArgs) in /Users/perry/git/space-station-14/RobustToolbox/Robust.Client/GameController/GameController.cs:line 521
   at Robust.Client.GameController.<StartupContinue>b__65_2(Object sender, FrameEventArgs args) in /Users/perry/git/space-station-14/RobustToolbox/Robust.Client/GameController/GameController.cs:line 264
   at Robust.Shared.Timing.GameLoop.Run() in /Users/perry/git/space-station-14/RobustToolbox/Robust.Shared/Timing/GameLoop.cs:line 187
   at Robust.Client.GameController.ContinueStartupAndLoop(DisplayMode mode) in /Users/perry/git/space-station-14/RobustToolbox/Robust.Client/GameController/GameController.Standalone.cs:line 163
   at Robust.Client.GameController.Run(DisplayMode mode, GameControllerOptions options, Func`1 logHandlerFactory) in /Users/perry/git/space-station-14/RobustToolbox/Robust.Client/GameController/GameController.Standalone.cs:line 133
   at Robust.Client.GameController.ParsedMain(CommandLineArgs args, Boolean contentStart, IMainArgs loaderArgs, GameControllerOptions options) in /Users/perry/git/space-station-14/RobustToolbox/Robust.Client/GameController/GameController.Standalone.cs:line 65
   at Robust.Client.GameController.Start(String[] args, GameControllerOptions options, Boolean contentStart, IMainArgs loaderArgs) in /Users/perry/git/space-station-14/RobustToolbox/Robust.Client/GameController/GameController.Standalone.cs:line 43
   at Robust.Client.ContentStart.Start(String[] args) in /Users/perry/git/space-station-14/RobustToolbox/Robust.Client/ContentStart.cs:line 10
   at Content.Client.Program.Main(String[] args) in /Users/perry/git/space-station-14/Content.Client/Program.cs:line 10