fixed a keyerror when passing a list into setBrush

- https://groups.google.com/forum/#!topic/pyqtgraph/xVyCC2f7gVo

Signed-off-by: Nicholas Tan Jerome <nicholas.jerome@kit.edu>

pyqtgraph/graphicsItems/ScatterPlotItem.py

@@ -443,7 +443,7 @@ class ScatterPlotItem(GraphicsObject):
             
         if len(args) == 1 and (isinstance(args[0], np.ndarray) or isinstance(args[0], list)):
             brushes = args[0]
-            if kargs['mask'] is not None:
+            if 'mask' in kargs and kargs['mask'] is not None:
                 brushes = brushes[kargs['mask']]
             if len(brushes) != len(dataSet):
                 raise Exception("Number of brushes does not match number of points (%d != %d)" % (len(brushes), len(dataSet)))