rez-gui: View Resolve Graph: TypeError

Hello,

I'm getting a TypeError trying to view the resolve graph through rez gui:

```
Traceback (most recent call last):
File ".../rez/2.71.0.4/python-3.7/lib/python3.7/site-packages/rezgui/widgets/ViewGraphButton.py", line 52, in _view_resolve_graph
view_graph(graph_str, self.window(), prune_to=self.package_name)
File ".../rez/2.71.0.4/python-3.7/lib/python3.7/site-packages/rezgui/dialogs/WriteGraphDialog.py", line 132, in view_graph
dlg = ImageViewerDialog(filepath, parent)
File ".../rez/2.71.0.4/python-3.7/lib/python3.7/site-packages/rezgui/dialogs/ImageViewerDialog.py", line 11, in __init__
super(ImageViewerDialog, self).__init__(parent)
TypeError: __init__() missing 2 required positional arguments: 'config' and 'config_key'
/tmp/rez_context_d79kw32h/rez-shell.sh: line 8: 281317 Aborted
```
rez: 2.71.0
python: 3.7.5
os: cent7
Qt: PyQt5 (with PySide2 the `View Resolve Graph` button does not respond at all?)

Must admit I'm opening rez-gui through some hacky python as our current rez-gui entry point script's shabang line breaks the environment I created for it:
```
rez env python-3 rez-2 Qtpy-1.2 PyQt-5.13 -c "\$PYTHON_EXE -c \"from rez.cli._main import run; run('gui')\""
```

Start with rezgui/widgets/ViewGraphButton.py and rezgui/dialogs/WriteGraphDialog.py, following the traceback into ImageViewerDialog.py. Reproduce the View Resolve Graph action with PyQt5 and, if possible, PySide2; done means the graph view no longer raises the reported TypeError or leaves the button unresponsive.

Written by the indexing model from the issue text.