Showing Event in Lead Dashboard Error

Hello there,
Trying to add Event To Lead Doc type Dashboard, any event have this Lead in Event Participants table supposed to be counted in Lead Dashboard;
I’ve used the following script to reach out that and override Lead Dashboard also I have added it to my app hooks file

from frappe import _
def get_data(data):
     return {
  		'fieldname': 'lead',
  		'non_standard_fieldnames': {
  			'Quotation': 'party_name',
  			'Opportunity': 'party_name',
            'Event': 'reference_docname'
  		'dynamic_links': {
  			'party_name': ['Lead', 'quotation_to'],
  		'transactions': [
  				'items': ['Opportunity', 'Quotation','Event']

but I got this Error:

TypeError: ‘str’ object does not support item assignment

what is the mistake here?
Any Response appreciated
Thanks in advance