Dashboard charts: cannot unpack non-iterable NoneType object

Created by: vic-t

The default dashboard shipped with ERPNext cannot be opened with the current Libracore fork.

The problem seems to be with chart type "custom" where the chart source is "account balance timeline".

Error report:

Traceback (most recent call last):
  File "/home/frappe/frappe-bench/apps/frappe/frappe/app.py", line 63, in application
    response = frappe.api.handle()
  File "/home/frappe/frappe-bench/apps/frappe/frappe/api.py", line 55, in handle
    return frappe.handler.handle()
  File "/home/frappe/frappe-bench/apps/frappe/frappe/handler.py", line 21, in handle
    data = execute_cmd(cmd)
  File "/home/frappe/frappe-bench/apps/frappe/frappe/handler.py", line 56, in execute_cmd
    return frappe.call(method, **frappe.form_dict)
  File "/home/frappe/frappe-bench/apps/frappe/frappe/__init__.py", line 1036, in call
    return fn(*args, **newargs)
  File "/home/frappe/frappe-bench/apps/frappe/frappe/core/page/dashboard/dashboard.py", line 27, in wrapper
    results = generate_and_cache_results(chart, chart_name, function, cache_key)
  File "/home/frappe/frappe-bench/apps/frappe/frappe/core/page/dashboard/dashboard.py", line 32, in generate_and_cache_results
    results = function(chart_name = chart_name)
  File "/home/frappe/frappe-bench/apps/erpnext/erpnext/accounts/dashboard_chart_source/account_balance_timeline/account_balance_timeline.py", line 37, in get
    gl_entries = get_gl_entries(account, get_period_ending(to_date, timegrain))
  File "/home/frappe/frappe-bench/apps/erpnext/erpnext/accounts/dashboard_chart_source/account_balance_timeline/account_balance_timeline.py", line 80, in get_gl_entries
    child_accounts = get_descendants_of('Account', account, ignore_permissions=True)
  File "/home/frappe/frappe-bench/apps/frappe/frappe/utils/nestedset.py", line 281, in get_descendants_of
    lft, rgt = frappe.db.get_value(doctype, name, ['lft', 'rgt'])
TypeError: cannot unpack non-iterable NoneType object