Throw an exception if the user tries to render a trace without stepping the simulation

Author: gaborszita

pyrtl/simulation.py

@@ -1756,6 +1756,8 @@ def render_trace(
             is not found in the map, the argument ``repr_func`` will be used instead.
         :param segment_size: Traces are broken in the segments of this number of cycles.
         """
+        if len(self) == 0:
+            raise PyrtlError("You need to step the simulation at least once to render a trace.")
         if repr_per_name is None:
             repr_per_name = {}
         if _currently_in_jupyter_notebook():