fixing plot_NN_2D.py for projects without mask values

scripts/plot_NN_2D.py

@@ -82,7 +82,7 @@ total_weights = c.w_test*np.array(c.class_weight)[c.y_test.astype(int)]
 
 try:
     mask_value = c.mask_value
-except NameError:
+except AttributeError:
     mask_value = None
 
 # varx_test = c.x_test[:,varx_index]