Why does this error occurs?

One common mistake is when you have an array of objects, you need to initialize the array AND loop through all the objects and initialize each element.

e.g.

AnyClass[] myArray;  // I want an array called myArray, and it will contain elements of type AnyClass

println(myArray[1].anyProperty); // this will give null pointer exception because myArray has not been created yet.

myArray = new AnyClass[100]; // now the myArray array has been created: enough memory to store 100x AnyClass instances has been allocated
// BUT still myArray[1].anyProperty will give null pointer exception because that instance of AnyClass as not been created

// so we need to loop through the array, and create an instance of AnyClass 100 times
for(int i=0; i<100; i++) myArray[i] = new AnyClass();

// now we can use myArray[...]