FIX: default color handling in bar3D

If color not specified (default to `None`), get the next color from the
axes color cycle.

closes #6989

lib/mpl_toolkits/mplot3d/axes3d.py

@@ -2443,9 +2443,9 @@ def bar3d(self, x, y, z, dx, dy, dz, color=None,
 
         facecolors = []
         if color is None:
-            # no color specified
-            facecolors = [None] * len(x)
-        elif len(color) == len(x):
+            color = [self._get_lines.get_next_color()]
+
+        if len(color) == len(x):
             # bar colors specified, need to expand to number of faces
             for c in color:
                 facecolors.extend([c] * 6)