This smells like there may be a corrupted file in your Bisq directory.
As for withdrawing your funds, if you are still able to access your private keys (is it the case ?), with those private keys you should be able to withdraw your funds using any wallet.
The first thing to do would be to backup the Bisq directory and all important data.
Do you have running trades, offers or dispute at the moment ?
If not, after having withdrawn your funds, you should remove your actual Bisq directory (but backup it before), and restart Bisq (which will recreate a new dir).
If you are not able to withdraw your funds, a second solution would also be to remove your actual Bisq directory (but backup it before), and restart Bisq (which will recreate a new dir), and then take your old wallet from the old directory, and copy it in the new directory.
In any case, do things carefully.
And perhaps wait for other advices. @ManfredKarrer is never very far.
Bisq directory locations:
Mac OSX: /Users/username/Library/Application Support/Bisq
Windows 7,8: C:\Documents and Settings\username\AppData\Roaming\Bisq
Windows XP: C:\Documents and Settings\username\Application Data\Bisq