Development version first run - ThemeManager no attribute '_write_theme'

[John Lines]

**OpenLP Bug Report**
Version: {'full': '2.5.dev2856', 'version': '2.5.dev2856', 'build': None}

--- Details of the Exception. ---

First run of Development version, on system which had/has the packaged Debian version, and existing 2.4 themes

 --- Exception Traceback ---
Traceback (most recent call last):
  File "./run_openlp.py", line 71, in <module>
    start()
  File "./run_openlp.py", line 67, in start
    main()
  File "/home/john/projects/openlp/openlp/trunk/openlp/core/app.py", line 450, in main
    sys.exit(application.run(qt_args))
  File "/home/john/projects/openlp/openlp/trunk/openlp/core/app.py", line 125, in run
    Registry().execute('bootstrap_initialise')
  File "/home/john/projects/openlp/openlp/trunk/openlp/core/common/registry.py", line 148, in execute
    result = function(*args, **kwargs)
  File "/home/john/projects/openlp/openlp/trunk/openlp/core/ui/thememanager.py", line 154, in bootstrap_initialise
    self.upgrade_themes() # TODO: Can be removed when upgrade path from OpenLP 2.4 no longer needed
  File "/home/john/projects/openlp/openlp/trunk/openlp/core/ui/thememanager.py", line 176, in upgrade_themes
    self._write_theme(theme)
AttributeError: 'ThemeManager' object has no attribute '_write_theme'

--- System information ---
Platform: Linux-4.19.0-5-amd64-x86_64-with-debian-10.0

--- Library Versions ---
Python: 3.7.3
PyQt5: 5.11.3
SQLAlchemy: 1.2.18
Alembic: 1.0.0
BeautifulSoup: 4.7.1
lxml: 4.3.3
Chardet: 3.0.4
PyEnchant: -
Mako: 1.0.7
VLC: OK
PyUNO: -

The upgraded had created a backup of my existing data in .local/share/

System runs without the crash if I remove the openlp directory in .local/share so it does not attempt an upgrade.