Fill combobox with NULL values in PyQGIS

Question

I'm writing a plugin that has a form with a combo box that I fill with the unique values in a field of a vector layer in the project.

The problem is that there are NULL values in that column and I get this error when I run the code:

TypeError: index 0 has type 'QVariant' but 'str' is expected

I'm trying with this code, but I still get the same error:

layer = QgsProject.instance().mapLayersByName('MyLayer')[0]
fieldsLayer = layer.fields()
idxField = fieldsLayer.indexFromName('MyField')
unique_values = layer.uniqueValues(idxField)

combovalues = []

for value in unique_values:
    if value == 'NULL':
        combovalues.append('')
    else:
        combovalues.append(value)

self.dlg.cmbRuta.addItems(combovalues)

Answer 1

'NULL' is not different from any string. Instead, you should use NULL. Remove apostrophes. NULL is automatically imported in QGIS. Outside QGIS you need to import NULL. It is a PyQGIS variant, not Python.

from qgis.core import NULL

...
...

for value in unique_values:
    if value == NULL:
        combovalues.append('')

....