Load map crashes with no maps/no folder

[CH4Code]

Short: When there are no maps present in the map folders the game crashes with an exception.

Reproduce
step 1: delete all maps from user/Openra/maps/mod/revision and programs/openra/mod/maps (except shellmaps).
step 2: press load map button in main menu

Noticed this while rebuilding a new mod with ModSDK on a second computer.

Expected: Load maps shows empty screen instead of crashing.

Exception Detail:

  Exception of type `System.InvalidOperationException`: Die Sequenz enthält kein übereinstimmendes Element.
     bei System.Linq.Enumerable.First[TSource](IEnumerable`1 source, Func`2 predicate)
   bei OpenRA.MapCache.ChooseInitialMap(String initialUid, MersenneTwister random) in e:\GitExt\OpenRAModSDK\engine\OpenRA.Game\Map\MapCache.cs:Zeile 333.
   bei OpenRA.Mods.Common.Widgets.Logic.MapChooserLogic..ctor(Widget widget, ModData modData, String initialMap, MapClassification initialTab, Action onExit, Action`1 onSelect, MapVisibility filter) in e:\GitExt\OpenRAModSDK\engine\OpenRA.Mods.Common\Widgets\Logic\MapChooserLogic.cs:Zeile 135.
...

[noamyogev84]

I would like to investigate it. Can I be assigned for this issue?

[GraionDilach]

@noamyogev84 Just file a PR, we don't use assignments much.

[noamyogev84]

@CH4Code @GraionDilach just pinging you guys to kindly ask for your observation on my PR

[GraionDilach]

I will probably find time to look into PRs tomorrow evening or Sunday. Sorry for the inconvenience.