Crash digitizing a feature when transaction group is on (after save)

[elpaso]

To reproduce: use the project and data from in https://github.com/qgis/QGIS/files/5348327/test.zip

1. Start editing
2. Digitize a feature (you get the error from Layer X: Feature has too many attributes (expecting 5, received 7) when "Automatic Creation of Transaction Group" is enabled #39230 but that's completely unrelated and the feature is saved)
3. Save the layer (but keep it in editing state) (under the hood the transaction is closed and set to nullptr)
4. digitize another feature, the crash happens when you click ok in the form (because transaction is nullptr)

CC @rouault

[elpaso]

Reproducible on PG and GPKG.

[elpaso]

Test case to reproduce the issue:

    def testTransactionGroupCrash(self):
        """Test issue GH #39265"""

        project = QgsProject()
        project.setAutoTransaction(True)
        tmpfile = os.path.join(
            self.basetestpath, 'tempGeoPackageTransactionCrash.gpkg')
        ds = ogr.GetDriverByName('GPKG').CreateDataSource(tmpfile)
        lyr = ds.CreateLayer('test', geom_type=ogr.wkbPoint)
        lyr.CreateField(ogr.FieldDefn('str_field', ogr.OFTString))

        f = ogr.Feature(lyr.GetLayerDefn())
        f.SetGeometry(ogr.CreateGeometryFromWkt('POINT (1 1)'))
        f.SetField('str_field', 'one')
        lyr.CreateFeature(f)

        del lyr
        del ds

        vl = QgsVectorLayer(tmpfile + '|layername=test', 'test', 'ogr')

        project.addMapLayers([vl])

        feature = next(vl.getFeatures())
        feature.setAttributes([None, 'two'])

        self.assertTrue(vl.startEditing())
        self.assertTrue(vl.addFeature(feature))

        # Save
        self.assertTrue(vl.commitChanges(False))

        # Now add another one
        feature.setAttributes([None, 'three'])
        self.assertTrue(vl.addFeature(feature))