do not raise error if no wannier function for the isosurface plot

[npaulish]

~/codes/git/aiidalab-qe-wannier90/src/aiidalab_qe_wannier90/result/result.py in on_single_row_select(self, change)
    185         key = f'aiida_{int(id):05d}'
    186         data = []
--> 187         if 'isovalue' in self.isosurface_data['parameters'][key]:
    188             for item in ['positive', 'negative']:
    189                 vertices = self.isosurface_data['mesh_data'][f'{key}_{item}_vertices'].value.tolist()

TypeError: 'NoneType' object is not subscriptable