don't crash

`drawFunc` should return an empty widget.

src/Hamming/App.hs

@@ -30,6 +30,7 @@ import Brick.AttrMap (forceAttrMap)
 import Brick.Main (App (..), halt, neverShowCursor)
 import Brick.Types (BrickEvent, EventM, Widget)
 import Brick.Util (style)
+import Brick.Widgets.Core (emptyWidget)
 
 import Hamming.App.Types
 
@@ -43,7 +44,7 @@ mainApp = App
   }
 
 drawFunc :: AppState -> [Widget ()]
-drawFunc = const []
+drawFunc = const [emptyWidget]
 
 eventHandler :: BrickEvent () () -> EventM () AppState ()
 eventHandler = const halt