ale_fix_on_save does not work while nerdtree is open

## Information

**VIM version**

VIM - Vi IMproved 8.2 (2019 Dec 12, compiled Apr 26 2020 17:02:50)
Included patches: 1-647

Operating System: Ubuntu 20.04

## What went wrong

I'm having the same issue that was described [here](https://github.com/dense-analysis/ale/issues/1066). I have `let g:ale_fix_on_save = 1` set, but ALEFix does not automatically run on save unless I close nerdtree. I can reliably reproduce this by toggling nerdtree on and off.

## Reproducing the bug

1. Setup vim + ale + nerdtree. My full vimrc can be found [here](https://github.com/tdickman/dotfiles/blob/master/vim/vimrc)
2. Configure vim to have `ale_fix_on_save` enabled. Fixers won't run as long as nerdtree is open.
3. Close nerdtree. Save file (fixers run).

### :ALEInfo

```
Current Filetype: python
Available Linters: ['bandit', 'flake8', 'mypy', 'prospector', 'pycodestyle', 'pydocstyle', 'pyflakes', 'pylama', 'pylint', 'pyls', 'pyre', 'vulture']
Enabled Linters: ['pylint']
Suggested Fixers:
'add_blank_lines_for_python_control_statements' - Add blank lines before control statements.
'autopep8' - Fix PEP8 issues with autopep8.
'black' - Fix PEP8 issues with black.
'isort' - Sort Python imports with isort.
'remove_trailing_lines' - Remove all blank lines at the end of a file.
'reorder-python-imports' - Sort Python imports with reorder-python-imports.
'trim_whitespace' - Remove all trailing whitespace characters at the end of every line.
'yapf' - Fix Python files with yapf.
Linter Variables:

let g:ale_python_auto_pipenv = 0
let g:ale_python_black_auto_pipenv = 0
let g:ale_python_black_change_directory = 1
let g:ale_python_black_executable = 'black'
let g:ale_python_black_options = ''
let g:ale_python_black_use_global = 0
let g:ale_python_pylint_auto_pipenv = 0
let g:ale_python_pylint_change_directory = 1
let g:ale_python_pylint_executable = 'pylint'
let g:ale_python_pylint_options = ''
let g:ale_python_pylint_use_global = 0
let g:ale_python_pylint_use_msg_id = 0
Global Variables:

let g:ale_cache_executable_check_failures = v:null
let g:ale_change_sign_column_color = 0
let g:ale_command_wrapper = ''
let g:ale_completion_delay = 100
let g:ale_completion_enabled = 1
let g:ale_completion_max_suggestions = 50
let g:ale_echo_cursor = 1
let g:ale_echo_msg_error_str = 'Error'
let g:ale_echo_msg_format = '%code: %%s'
let g:ale_echo_msg_info_str = 'Info'
let g:ale_echo_msg_warning_str = 'Warning'
let g:ale_enabled = 1
let g:ale_fix_on_save = 1
let g:ale_fixers = {'javascriptreact': ['eslint'], 'javascript': ['eslint'], 'python': ['black']}
let g:ale_history_enabled = 1
let g:ale_history_log_output = 1
let g:ale_keep_list_window_open = 0
let g:ale_lint_delay = 200
let g:ale_lint_on_enter = 1
let g:ale_lint_on_filetype_changed = 1
let g:ale_lint_on_insert_leave = 1
let g:ale_lint_on_save = 1
let g:ale_lint_on_text_changed = 'normal'
let g:ale_linter_aliases = {}
let g:ale_linters = {'javascriptreact': ['eslint'], 'javascript': ['eslint'], 'python': ['pylint']}
let g:ale_linters_explicit = 0
let g:ale_list_vertical = 0
let g:ale_list_window_size = 10
let g:ale_loclist_msg_format = '%code: %%s'
let g:ale_lsp_root = {}
let g:ale_max_buffer_history_size = 20
let g:ale_max_signs = -1
let g:ale_maximum_file_size = v:null
let g:ale_open_list = 0
let g:ale_pattern_options = v:null
let g:ale_pattern_options_enabled = v:null
let g:ale_set_balloons = 0
let g:ale_set_highlights = 1
let g:ale_set_loclist = 1
let g:ale_set_quickfix = 0
let g:ale_set_signs = 1
let g:ale_sign_column_always = 0
let g:ale_sign_error = '>>'
let g:ale_sign_info = '--'
let g:ale_sign_offset = 1000000
let g:ale_sign_style_error = '>>'
let g:ale_sign_style_warning = '--'
let g:ale_sign_warning = '--'
let g:ale_sign_highlight_linenrs = 0
let g:ale_statusline_format = v:null
let g:ale_type_map = {}
let g:ale_use_global_executables = v:null
let g:ale_virtualtext_cursor = 0
let g:ale_warn_about_trailing_blank_lines = 1
let g:ale_warn_about_trailing_whitespace = 1
Command History:

(executable check - success) REDACTED

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Your code has been rated at 9.90/10 (previous run: 9.90/10, +0.00)

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Your code has been rated at 9.90/10 (previous run: 9.90/10, +0.00)

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Your code has been rated at 9.90/10 (previous run: 9.90/10, +0.00)

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```