HW：Java程序生成NullPointerException错误消息

I am writing code for my Data Structures class. My code consists of 5 classes that combine to create an array of 100 random integers of value between 0 and 100, pick out various statistical information from those numbers and generate the standard deviation compared to a normal distribution. Apparently, my method for calculating the standard deviation is linked to something the gives a null pointer error: I am writing code for my Data Structures class.