It could be the clutch being out of adjustment and not fully releasing.
Another possibility is that the neutral safety switches may be responsible. Make sure your kickstand is up before you put the bike in gear. If that's not the problem, make sure the plug is secure that attaches to the neutral switch on the clutch lever or master cylinder. If that checks out, look at the wiring on the neutral switch attached to the kickstand. If this is loose, it could also be the problem.