fixed bug in CenteredNorm, issue matplotlib#19972

Author: jpmattern

lib/matplotlib/colors.py

@@ -1359,6 +1359,8 @@ def __init__(self, vcenter=0, halfrange=None, clip=False):
             array([0.25, 0.5 , 1.  ])
         """
         self._vcenter = vcenter
+        self.vmin = None
+        self.vmax = None
         # calling the halfrange setter to set vmin and vmax
         self.halfrange = halfrange
         self.clip = clip