Set instance pointer to nullptr in destroy()

Useful for tests

src/dev/ParallelChecker.cpp

@@ -14,4 +14,5 @@ ParallelChecker::destroy()
 {
   Assert(ParallelChecker::m_instance != nullptr, "ParallelChecker has already been destroyed");
   delete ParallelChecker::m_instance;
+  ParallelChecker::m_instance = nullptr;
 }