Fixed crash on no args to AddInputCallback. Clear steps for all players when deleting on ScreenEditMenu. Changed assert message in ApplyDerivedType to be useful. Fixed typos in OptionRowHandler message and EditHelpText.

src/ScreenEditMenu.cpp

@@ -81,7 +81,10 @@ void ScreenEditMenu::HandleScreenMessage( const ScreenMessage SM )
 
 		Song* pSong = GAMESTATE->m_pCurSong;
 		Steps* pStepsToDelete = GAMESTATE->m_pCurSteps[PLAYER_1];
-		GAMESTATE->m_pCurSteps[PLAYER_1].Set(NULL);
+		FOREACH_PlayerNumber(pn)
+		{
+			GAMESTATE->m_pCurSteps[pn].Set(NULL);
+		}
 		bool bSaveSong = !pStepsToDelete->WasLoadedFromProfile();
 		pSong->DeleteSteps( pStepsToDelete );
 		SONGMAN->Invalidate( pSong );