fix the unit test 'SolveFactoryTest' which failed due to the added out of...

Fix the unit test 'SolveFactoryTest' which failed due to an access of non-allocated memory which is observed due the added out of bounds check; additionally fixed the test 'VariableRFCavityFringeFieldTest.TestConstructor which crashed during run time

/usr/include/c++/9/bits/shared_ptr_base.h:1007: std::__shared_ptr_access<_Tp, _Lp, <anonymous>, <anonymous> >::element_type& std::__shared_ptr_access<_Tp, _Lp, <anonymous>, <anonymous> >::operator*() const [with _Tp = endfieldmodel::EndFieldModel; __gnu_cxx::_Lock_policy _Lp = __gnu_cxx::_S_atomic; bool <anonymous> = false; bool <anonymous> = false; std::__shared_ptr_access<_Tp, _Lp, <anonymous>, <anonymous> >::element_type = endfieldmodel::EndFieldModel]: Assertion '_M_get() != nullptr' failed.

Closes NightlyBuild#28 (closed)