I checked the kick stand switch
Are you putting it in gear with the kickstand up or down?
Is it stalling as if it is being shut down electrically by the ignition switch, or is it stalling with a great big lurch forward?
It's not unusual for clutch plates to seize together when they are clamped together for a long time (e.g. during storage).
Use to happen to my ex500 to combat this problem, I increased the idle knob to around 4k after that it would get into 1st gear without the engine stalling. I think it was partly due to sitting for a long time/ cold. What bike do you ride?
i am riding a klx250s