plot_polar_smooths() fails if there are tensor product smooths/interactions and discrete = TRUE

[stefanocoretta]

If the polar GAM model has tensor product smooths/interactions and discrete = TRUE, the function plot_polar_smooths() returns an error and a warning:

Error: object 'X' not found
In addition: Warning message:
In predict.bamd(object, newdata, type, se.fit, terms, exclude, block.size,  :
  exclude ignored by discrete prediction at present