You are missing the important 'mocks initialization' statement in your before method. Adding this statement should solve the problem.

MockitoAnnotations.initMocks(this); // for below Mockito 3.4.0

MockitoAnnotations.openMocks(this); // for Mockito 3.4.0 and above

Sources :

1. link : Initialising mock objects - Mockito

2. link : https://www.javadoc.io/doc/org.mockito/mockito-core/3.4.0/org/mockito/MockitoAnnotations.html