Jupyter_viz: Allow measures to be None (#2163)

Currently measures crashed when it's None (which is the default). The only way to circumvent this is adding [], but that's non-ideal and not obvious.

This PR fixes that, so that measures can actually be the default value of None.

This also proves we need to test Jupyter viz in notebooks, not only in console, since both have different code paths.

Author: EwoutH

mesa/visualization/jupyter_viz.py

@@ -143,14 +143,15 @@ def render_in_jupyter():
             # otherwise, do nothing (do not draw space)
 
             # 5. Plots
-            for measure in measures:
-                if callable(measure):
-                    # Is a custom object
-                    measure(model)
-                else:
-                    components_matplotlib.PlotMatplotlib(
-                        model, measure, dependencies=dependencies
-                    )
+            if measures:
+                for measure in measures:
+                    if callable(measure):
+                        # Is a custom object
+                        measure(model)
+                    else:
+                        components_matplotlib.PlotMatplotlib(
+                            model, measure, dependencies=dependencies
+                        )
 
     def render_in_browser():
         # if space drawer is disabled, do not include it