has_add_permission in admin.TabularInline gives KeyError exception

[Harm Verhagen]

I noticed that using {{has_add_permission()}} on a admin.TabularInline fails.

See the simple example below.

h3. steps to reproduce

models.py
{code}
class ModelA(models.Model):

name = models.CharField(max_length=2)
bs = models.ManyToManyField("ModelB", through="Through")

def str(self):

return "%s" % self.name

class ModelB(models.Model):

name = models.CharField(max_length=2)

def str(self):

return "%s" % self.name

class Through(models.Model):

a = models.ForeignKey(ModelA, on_delete=models.CASCADE)
b = models.ForeignKey(ModelB, on_delete=models.CASCADE)

def str(self):

return "%s-%s" % (self.a, self.b)

{code}

admin.py
{code}
class ThroughInline(admin.TabularInline):

model = Through

def has_add_permission(self, request, obj):

# setting this to False results in an error
return False

class ModelAAdmin(admin.ModelAdmin):

inlines = (ThroughInline, )

class ModelBAdmin(admin.ModelAdmin):

inlines = (ThroughInline, )

class ThroughAdmin(admin.ModelAdmin):

pass

{code}

Then create your objects.
{code}

from <yourapp>.models import ModelA, ModelB, Through
a = ModelA.objects.create(name='a')
b= ModelB.objects.create(name='b')
t = Through.objects.create(a=a, b=b)

{code}

Fire up a runserver and visit the admin api: ​http://localhost:8000/admin/<yourapp>/modela/1/change/

h3. result

A server 500
{code}
KeyError at /admin/<appname>/modela/1/change/

Request Method: GET
Request URL: ​http://localhost:8000/admin/<appname>/modela/1/change/
Django Version: 2.1
Exception Type: KeyError
Exception Value: 'b'

Template error:
In template ...contrib/admin/templates/admin/edit_inline/tabular.html, error at line 13

b
3 : data-inline-type="tabular"
4 : data-inline-formset="{{ inline_admin_formset.inline_formset_data }}">
5 : <div class="tabular inline-related {% if forloop.last %}last-related{% endif %}">
6 : {{ inline_admin_formset.formset.management_form }}
7 : <fieldset class="module {{ inline_admin_formset.classes }}">
8 : <h2>{{ inline_admin_formset.opts.verbose_name_plural|capfirst }}</h2>
9 : {{ inline_admin_formset.formset.non_form_errors }}
10 : <table>
11 : <thead><tr>
12 : <th class="original"></th>
13 : {% for field in inline_admin_formset.fields %}
14 : {% if not field.widget.is_hidden %}
15 : <th{% if field.required %} class="required"{% endif %}>{{ field.label|capfirst }}
16 : {% if field.help_text %}&nbsp;<img src="{% static "admin/img/icon-unknown.svg" %}" class="help help-tooltip" width="10" height="10" alt="({{ field.help_text|striptags }})" title="{{ field.help_text|striptags }}">{% endif %}
17 : </th>
18 : {% endif %}
19 : {% endfor %}
20 : {% if inline_admin_formset.formset.can_delete %}<th>{% trans "Delete?" %}</th>{% endif %}
21 : </tr></thead>
22 :
23 : <tbody>

Traceback:

File "/venv3/lib/python3.5/site-packages/django/core/handlers/exception.py" in inner

34. response = get_response(request)

File "/venv3/lib/python3.5/site-packages/django/core/handlers/base.py" in _get_response

156. response = self.process_exception_by_middleware(e, request)

File "/venv3/lib/python3.5/site-packages/django/core/handlers/base.py" in _get_response

154. response = response.render()

File "/venv3/lib/python3.5/site-packages/django/template/response.py" in render

106. self.content = self.rendered_content

File "/venv3/lib/python3.5/site-packages/django/template/response.py" in rendered_content

83. content = template.render(context, self._request)

{code}

h3. expected result

The inlines are displayed without the option to create new Through instances.

h3. version
{{Django==2.1}}