Spring bean field (which is instantiated explicitly) is assigned to null sometimes

Question

One day we faced an incident that application started crashing because of the NPE. I found the place where it was crashing, but I cannot be sure why does it happen.

Roughly speaking lets assume that I have a class A which appears a spring bean at the same time. This class has a field property which is instantiated explicitly (yes, I know how it sounds like) and a method propertyUser which uses this field:

class A {
   private ObjectClass property = new ObjectClass();
   public void propertyUser() {
      property.action();
   }   
   ....
   getter\setter for property
}

And after some time of usual work, property field has become null which has resulted in NPE from propertyUser method. I changed this legacy code and now this property is assigned via spring container either, however I do need to be sure that this was the reason of this error appearance.

So I would be very pleased if somebody could give me a hint.