Don't import wagtailmedia models if it's not in INSTALLED_APPS

[kaedroho]

If you have wagtailmedia installed with pip, but not in INSTALLED_APPS, Django will raise an error on startup:

Traceback (most recent call last):
  File "/home/gitpod/.pyenv/versions/3.8.12/lib/python3.8/threading.py", line 932, in _bootstrap_inner
    self.run()
  File "/home/gitpod/.pyenv/versions/3.8.12/lib/python3.8/threading.py", line 870, in run
    self._target(*self._args, **self._kwargs)
  File "/workspace/.pip-modules/lib/python3.8/site-packages/django/utils/autoreload.py", line 64, in wrapper
    fn(*args, **kwargs)
  File "/workspace/.pip-modules/lib/python3.8/site-packages/django/core/management/commands/runserver.py", line 110, in inner_run
    autoreload.raise_last_exception()
  File "/workspace/.pip-modules/lib/python3.8/site-packages/django/utils/autoreload.py", line 87, in raise_last_exception
    raise _exception[1]
  File "/workspace/.pip-modules/lib/python3.8/site-packages/django/core/management/__init__.py", line 375, in execute
    autoreload.check_errors(django.setup)()
  File "/workspace/.pip-modules/lib/python3.8/site-packages/django/utils/autoreload.py", line 64, in wrapper
    fn(*args, **kwargs)
  File "/workspace/.pip-modules/lib/python3.8/site-packages/django/__init__.py", line 24, in setup
    apps.populate(settings.INSTALLED_APPS)
  File "/workspace/.pip-modules/lib/python3.8/site-packages/django/apps/registry.py", line 122, in populate
    app_config.ready()
  File "/workspace/gitpod-wagtail-develop/wagtail-grapple/grapple/apps.py", line 12, in ready
    from .actions import import_apps, load_type_fields
  File "/workspace/gitpod-wagtail-develop/wagtail-grapple/grapple/actions.py", line 42, in <module>
    from wagtailmedia.models import AbstractMedia
  File "/workspace/.pip-modules/lib/python3.8/site-packages/wagtailmedia/models.py", line 157, in <module>
    class Media(AbstractMedia):
  File "/workspace/.pip-modules/lib/python3.8/site-packages/django/db/models/base.py", line 113, in __new__
    raise RuntimeError(
RuntimeError: Model class wagtailmedia.models.Media doesn't declare an explicit app_label and isn't in an application in INSTALLED_APPS.

This PR adds a check to see if wagtailmedia is in INSTALLED_APPS before trying to import its models.

[zerolab]

There are a couple more places that would be good to add extra guards:

diff --git a/grapple/models.py b/grapple/models.py
index 4b57479..706739b 100644
--- a/grapple/models.py
+++ b/grapple/models.py
@@ -121,15 +121,6 @@ def GraphQLDocument(field_name: str, **kwargs):
     return Mixin
 
 
-def GraphQLMedia(field_name: str, **kwargs):
-    def Mixin():
-        from .types.media import get_media_type
-
-        return GraphQLField(field_name, get_media_type, **kwargs)
-
-    return Mixin
-
-
 def GraphQLPage(field_name: str, **kwargs):
     def Mixin():
         from .types.pages import PageInterface
@@ -211,3 +202,13 @@ def GraphQLTag(field_name: str, **kwargs):
         return GraphQLField(field_name, TagObjectType, **kwargs)
 
     return Mixin
+
+
+if apps.is_installed("wagtailmedia"):
+    def GraphQLMedia(field_name: str, **kwargs):
+        def Mixin():
+            from .types.media import get_media_type
+
+            return GraphQLField(field_name, get_media_type, **kwargs)
+
+        return Mixin
\ No newline at end of file
diff --git a/grapple/wagtail_hooks.py b/grapple/wagtail_hooks.py
index b7049fc..d5e9d57 100644
--- a/grapple/wagtail_hooks.py
+++ b/grapple/wagtail_hooks.py
@@ -1,3 +1,4 @@
+from django.apps import apps
 from graphene import ObjectType
 
 try:
@@ -34,12 +35,10 @@ def register_schema_query(query_mixins):
         RedirectsQuery,
     ]
 
-    try:
+    if apps.is_installed("wagtailmedia"):
         from .types.media import MediaQuery
 
         query_mixins.append(MediaQuery())
-    except ModuleNotFoundError:
-        pass
 
     query_mixins += registry.schema
 

[zerolab]

Applied the changes and merge manually. Thanks @kaedroho