Describe the bug
When an administrator deletes a meeting with proposals related, there is an error on proposal filtering.

To Reproduce
Steps to reproduce the behavior:

1. Create a meeting
2. Create a proposal that comes from this meeting
3. Go to admin panel, delete the meeting
4. Go to proposals list filtering
5. See error

Expected behavior
When deleting, I should see a confirm dialog saying:
You're going to also delete a proposal: "yyyyy". Are you sure?

If there are X proposals then it should say:

You're going to also delete X proposals: 
* YYYYY
* ZZZZZ
Are you sure? 

And when the meeting is delete we should also delete this proposal.

Screenshots

Stacktrace

app_1 |
app_1 |
app_1 | ActionView::Template::Error (uninitialized constant NilClassPresenter):
app_1 | 1: <%= card_for proposal, from: proposal %>
app_1 |
app_1 | activesupport (5.2.4.3) lib/active_support/inflector/methods.rb:283:in const_get' app_1 | activesupport (5.2.4.3) lib/active_support/inflector/methods.rb:283:in block in constantize'
app_1 | activesupport (5.2.4.3) lib/active_support/inflector/methods.rb:281:in each' app_1 | activesupport (5.2.4.3) lib/active_support/inflector/methods.rb:281:in inject'
app_1 | activesupport (5.2.4.3) lib/active_support/inflector/methods.rb:281:in constantize' app_1 | activesupport (5.2.4.3) lib/active_support/core_ext/string/inflections.rb:68:in constantize'

Extra data (please complete the following information):

• Device: Desktop
• Device OS: Ubuntu 20.04
• Browser: Firefox / Any
• Decidim Version: develop
• Decidim installation: localhost. First detected at decidim.barcelona (v0.21)

Additional context
N/A

Acceptance criteria

• As an administrator when I delete a meeting with proposals related, I see a confirmation message when deleting it the meeting.
• As an administrator when I delete a meeting with proposals related, I see the proposals titles on this confirmation meesage
• When the meeting is deleted the proposals are also deleted
• As a visitor I can see the proposals listing without any exception