[NL/TimeStepping] Check if (timestep, delta_t) vector is valid

NumLib/TimeStepping/Algorithms/CreateFixedTimeStepping.cpp

@@ -49,6 +49,12 @@ std::unique_ptr<TimeStepAlgorithm> createFixedTimeStepping(
             //! \ogs_file_param{prj__time_loop__processes__process__time_stepping__FixedTimeStepping__timesteps__pair__delta_t}
             pair.getConfigParameter<double>("delta_t"));
     }
+    if (!FixedTimeStepping::areRepeatDtPairsValid(repeat_dt_pairs))
+    {
+        OGS_FATAL(
+            "CreateFixedTimeStepping: invalid specification of (repeat, "
+            "delta_t) pairs");
+    }
 
     return std::make_unique<FixedTimeStepping>(
         t_initial, t_end, repeat_dt_pairs, fixed_times_for_output);