Nothing is wrong with the bike. When the transmission is stopped, (the bike is not moving), even with the engine running, it's common for the gear teeth to not match up if you attempt to shift, (especially downshift), while stopped.
Proper technique is to down shift as your speed decreases. This allows you to be in the correct power band if you should need to accelerate due to traffic. Practice this and it will become habit. Normally people do not down shift to 1st while stopping, but just to second, then into neutral as they stop, or to 1st when they are stopped fully, then holding the clutch in while in first so as to be prepared for the light change, or unusual circumstances. Neutral or 1st at a stop light is a preference. In neutral you have less of a chance of getting out of the way should someone be endangering you.
Just braking to a stop, then attempting to down shift all the way to neutral is not going to hurt anything, but you will find that after one gear down, you may need to slightly engage the clutch, just enough to allow the gears to line up for the next shift. The bike should not need to move to do this, just let up a tad on the lever, then you will hear it engage the gear, squeeze the lever back in all the way and downshift again. Repeat as needed.
The "switching off" is called stalling. Too high a gear for the speed you are traveling usually, though too low a gear will lock the tire and sometimes stall as well. Practice rev matching the engine to the gear you are moving to when down shifting. A small blip of the throttle as you clutch in and down shift. It will take practice and some feel before you get it right, but it's much easier on the transmission and clutch, as well as placing you in better control while downshifting.
Good luck and please be careful.
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