Edit topology network

I have two SHP files in QGIS project (points: NODES.shp and lines: BRANCH.shp). NODES.shp has one ID attribute NODE_CON. BRANCH.shp has attributes NODE1 and NODE2 which represent start and end node of topology network.

I need the tool wich at inserting feature into BRANCH.shp automaticly detect NODE_CON of the start node of the new polyline and write it into NODE1. Similar it need to detect NODE_CON of the end node and write it into NODE2.

My idea:
I am trying with Default value on BRANCH form. My idea is to write the function getNodeCon with three parameters:

• layer_name,
• field_id and
• geometry of point.

I would call the function like that:
getNodeCon('NODES', 'NODE_CON', start_point($geometry)).

———function definition start—————

import os
from qgis.core import *
from qgis.gui import *


@qgsfunction(args=3, group='Custom')
def getNodeCon(layer_name, conn_field, tocka):

    layerList = QgsProject.instance().mapLayersByName(layer_name)
    layer = layerList[0]

    NodeCon = 'NOK'
   
    for feat in layer.getFeatures():
            if feat.geometry().within.tocka.geometry().buffer(0.5,100):
                    NodeCon = feat.fieldNameIndex(conn_field)
                    break
    return NodeCon

———function definition end—————

I get error measage:

Eval Error: QgsProject.mapLayersByName(): argument 1 has unexpected type 'list'