Ensure WidgetFilter sync_with_url uses Widget parameter to serialize values

[philippjfr]

Since Filter.value is of type Parameter it does not define the correct serialize methods for the Location sync to correct serialize and deserialize the data values. Therefore we now sync the Widget.value rather than the Filter.value allowing the value type of the parameter to provide the correct serialization. In future Location.sync in Panel should provide a dictionary of serializers and deserializers so we can override these directly.

Fixes #455

[hoxbro]

The error you are seeing is because you are syncing twice with the widgets.

[philippjfr]

You sure? That's what the self.widget is not None check is for.

[hoxbro]

Isn't it called once in Filter and once in each widget?

[philippjfr]

Yes but in Filter self.widget is undefined so it's skipped.

[hoxbro]

You are right.

[philippjfr]

The problem really is just that the widget (unlike the Filter) always has a default value, so it gets synced, even if it's an empty list.

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