Only set colors if x_axis_key is string

Author: dstansby

Diff for: src/napari_matplotlib/histogram.py

@@ -209,10 +209,12 @@ def draw(self) -> None:
         # get the colormap from the layer depending on its type
         if isinstance(self.layers[0], napari.layers.Points):
             colormap = self.layers[0].face_colormap
-            self.layers[0].face_color = self.x_axis_key
+            if self.x_axis_key:
+                self.layers[0].face_color = self.x_axis_key
         elif isinstance(self.layers[0], napari.layers.Vectors):
             colormap = self.layers[0].edge_colormap
-            self.layers[0].edge_color = self.x_axis_key
+            if self.x_axis_key:
+                self.layers[0].edge_color = self.x_axis_key
         else:
             colormap = None