U can't use the expression  for electric potential energy of a uniformly charged sphere (i.e. P= 3Q²/20R) in case of a point mass. Carefully observe the 2nd-last step in the derivation of this expression:

 

                P = 3Q²/4R⁶ ._0^R x⁴dx

 

In case of point charge R=0. Thus the expression takes the 0/0 form which is meaningless.