There is something missing within .minecraft folder in Roaming, type %appdata%
Delete your .minecraft folder. (Make sure to make a copy of any of your saved worlds)
After you've deleted the .minecraft folder launch the Minecraft launcher.
This will regenerate with .minecraft folder and the error should be corrected.
If this error persists, comment this so I see and I'll have another look but I'm 90% sure that's what you need to do.
To save your worlds, click start, type %appdata% > .minecraft > saves
or follow this path C:\Users\"username"\AppData\Roaming.minecraft\saves
Once in saves click with your mouse and drag over all the folders inside the "saves" folder, hold Ctrl and press C while holding control.
Then go to a location you can temporarily store them, desktop is easiest as you can access it quickest.
When on your desktop hold Ctrl and press V while holding Ctrl, the saves are now on your desktop.
Then just delete the .minecraft folder and open the Minecraft launcher, it will give you a fresh .minecraft and will work now.
Remember once the fresh .minecraft has been made, close Minecraft, goto the .minecraft and copy your saves back into the saves folder the same way you copied them out.
(If you typed %appdata% in the start menu search, ignore this.)
Your roaming folder may not actually be visible as there is a setting that windows have to protect important folders, if you can't see the Appdata folder if you followed the path I said. Do this
- Type folder options after clicking the start button
- click folder options (it should appear in the results)
- It will open folder options, click the tab "view"
- Look for "hidden files and folders", below that it will say
"show...." or "hide...."
- Click show, click apply then ok
- It will now be visible.