this is due to because the wing tip portion lies at contact 
with the atmosphere air n so the pressure created at the 
tip is not uniform and thus resulting stall in that 
portion,for preventing that winglets can be installed at 
that portion.

Usually,stalls occur at high AOA or at low speeds.This is 
because the airflow over the wing just stops creating a 
very low lift.
 In the case of a swept wing,as we move towards the tip,the 
surface area of the wing can be considered to be decreasing.
Therefore, lower surface area means lower lift and thats 
why stalls start at wingtips.
 For your information, winglets prevent wingtip 
vortices.They just stop the pressure leak between the upper 
and lower surfaces of the wing thereby increaing the 
efficiency. It has a low effect on stalls.