Don't use osu! mode parser for other game modes for now (convert still works).

osu.Game.Modes.Catch/CatchRuleset.cs

@@ -24,6 +24,6 @@ namespace osu.Game.Modes.Catch
 
         public override ScoreProcessor CreateScoreProcessor(int hitObjectCount) => null;
 
-        public override HitObjectParser CreateHitObjectParser() => new OsuHitObjectParser();
+        public override HitObjectParser CreateHitObjectParser() => new NullHitObjectParser();
     }
 }

osu.Game.Modes.Mania/ManiaRuleset.cs

@@ -25,6 +25,6 @@ namespace osu.Game.Modes.Mania
 
         public override ScoreProcessor CreateScoreProcessor(int hitObjectCount) => null;
 
-        public override HitObjectParser CreateHitObjectParser() => new OsuHitObjectParser();
+        public override HitObjectParser CreateHitObjectParser() => new NullHitObjectParser();
     }
 }

osu.Game.Modes.Taiko/TaikoRuleset.cs

@@ -25,6 +25,6 @@ namespace osu.Game.Modes.Taiko
 
         public override ScoreProcessor CreateScoreProcessor(int hitObjectCount) => null;
 
-        public override HitObjectParser CreateHitObjectParser() => new OsuHitObjectParser();
+        public override HitObjectParser CreateHitObjectParser() => new NullHitObjectParser();
     }
 }

osu.Game/Modes/Objects/NullHitObjectParser.cs

@@ -0,0 +1,14 @@
+// Copyright (c) 2007-2017 ppy Pty Ltd <contact@ppy.sh>.
+// Licensed under the MIT Licence - https://raw.githubusercontent.com/ppy/osu/master/LICENCE
+
+
+namespace osu.Game.Modes.Objects
+{
+    /// <summary>
+    /// Returns null HitObjects but at least allows us to run.
+    /// </summary>
+    public class NullHitObjectParser : HitObjectParser
+    {
+        public override HitObject Parse(string text) => null;
+    }
+}

osu.Game/osu.Game.csproj

@@ -77,6 +77,7 @@
     <Compile Include="Graphics\UserInterface\TwoLayerButton.cs" />
     <Compile Include="Modes\Objects\Drawables\IDrawableHitObjectWithProxiedApproach.cs" />
     <Compile Include="Modes\Objects\HitObjectParser.cs" />
+    <Compile Include="Modes\Objects\NullHitObjectParser.cs" />
     <Compile Include="Modes\Score.cs" />
     <Compile Include="Modes\ScoreProcesssor.cs" />
     <Compile Include="Modes\UI\HealthDisplay.cs" />