I faced the same issue. Debugger started behaving this way all of a sudden, without me modifying any configuration or anything that may be significant. Fixed it by disabling Breakpoint on NullPointerException, though not sure how it got enabled or if it was enabled earlier as well.
Go to View Breakpoints (⌘ + ⇧ + F8) > Java Exception Breakpoints > java.lang.NullPointerException > uncheck Enabled.
If there is a legit NPE in the Unit Test, it works as usual.
