Yamaha YBR125 2005 Gears Slipping ??

[Cruiser420]

So i've been having a little trouble with the bike i've recently bought it didn't really happen much at first and i am a new rider still getting the hang of the gears but this is something else...

It's slipping from 2nd to neutral at speed and i'm not even holding the clutch, it's also very tricky sometimes not going up into neutral from first obviously i'm a new rider but at first this was never a problem for me and on my CBT bike never did this either.

Couple of things worth mentioning the chain came off.... So luckily that didn't go horribly wrong i've tightened the chain now and still having this problem, could this be a wheel alignment issue or sprocket related i'm thinking the gear box may need attention??

Any help would be great i'm a total newbie