Ensure MapCache disposes cleanly.

This prevents the map loading thread writing to disposed objects which can have unintended side effects.

OpenRA.Game/Map/MapCache.cs

@@ -249,8 +249,21 @@ namespace OpenRA
 		}
 
 		public void Dispose()
+		{
+			if (previewLoaderThread == null)
 			{
 				sheetBuilder.Dispose();
+				return;
+			}
+
+			// We need to let the loader thread exit before we can dispose our sheet builder.
+			// Ideally we should dispose our resources before returning, but we don't to block waiting on the loader thread to exit.
+			// Instead, we'll queue disposal to be run once it has exited.
+			ThreadPool.QueueUserWorkItem(_ =>
+			{
+				previewLoaderThread.Join();
+				Game.RunAfterTick(sheetBuilder.Dispose);
+			});
 		}
 	}
 }