Fix/aplot index error (#1691)

* Avoiding error

* Using max

* Avoiding coverage issues

Author: germa89

src/ansys/mapdl/core/mapdl.py

@@ -1566,9 +1566,10 @@ def aplot(
 
             # individual surface isolation is quite slow, so just
             # color individual areas
-            if color_areas:
+            if color_areas:  # pragma: no cover
                 anum = surf["entity_num"]
-                rand = np.random.random(anum[-1] + 1)
+                size_ = max(anum) + 1
+                rand = np.random.random(size_)
                 area_color = rand[anum]
                 meshes.append({"mesh": surf, "scalars": area_color})
             else: