Handle no y-range maxmin output (seems like bug?)

piker/ui/_interaction.py

@@ -731,7 +731,11 @@ class ChartView(ViewBox):
                         )
 
         if set_range:
-            ylow, yhigh = self._maxmin()
+            yrange =  self._maxmin()
+            if yrange is None:
+                return
+
+            ylow, yhigh = yrange
 
             # view margins: stay within a % of the "true range"
             diff = yhigh - ylow