Whenever I try to open a singleplayer world it begins to load the world then suddenly it stops, then comes up with an error saying: internal Exception: java.lang.ClassCastException: bit cannot cast to fm. This is getting really annoying because the only thing I can do is play multiplayer (which gets boring after a while). In the minecraft launcher it says: [08:39:03 INFO]: Client> [08:39:03] [Server thread/INFO]: xXGTAproXx[local:E:2f7c33a4] logged in with entity id 0 at (410.5, 11.61503548491692, 856.5)
[08:39:03 INFO]: Client> [08:39:03] [Server thread/INFO]: xXGTAproXx joined the game
[08:39:06 INFO]: Client> [08:39:06] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 2734ms behind, skipping 54 tick(s)
[08:39:06 INFO]: Client> [08:39:06] [Server thread/INFO]: xXGTAproXx lost connection: TextComponent{text='Disconnected', siblings=[], style=Style{hasParent=false, color=null, bold=null, italic=null, underlined=null, obfuscated=null, clickEvent=null, hoverEvent=null}}
[08:39:06 INFO]: Client> [08:39:06] [Server thread/INFO]: xXGTAproXx left the game
[08:39:07 INFO]: Client> [08:39:07] [Server thread/INFO]: Stopping singleplayer server as player logged out
[08:39:07 INFO]: Client> [08:39:07] [Server thread/INFO]: Stopping server
[08:39:07 INFO]: Client> [08:39:07] [Server thread/INFO]: Saving players
[08:39:07 INFO]: Client> [08:39:07] [Server thread/INFO]: Saving worlds
[08:39:07 INFO]: Client> [08:39:07] [Server thread/INFO]: Saving chunks for level 'Never Ending PvP'/Overworld
[08:39:07 INFO]: Client> [08:39:07] [Server thread/INFO]: Saving chunks for level 'Never Ending PvP'/Nether
[08:39:07 INFO]: Client> [08:39:07] [Server thread/INFO]: Saving chunks for level 'Never Ending PvP'/The End
One last thing that I found rather interesting was that when I push Back To Title Screen, it sends me to the multiplayer menu. When I was just in the Singleplayer menu. This bug does not happen in 1.6.4.

Thanks for your help!