text: Title: [BUG] Adding model to existing plugin, existing plugin instances disappear
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  ## Description
  
  I have some simple plugins which are basically just templates to wrap child content, things like rows and columns. Usually they don't need a model. If the requirements of a plugin change and I do need a model for a plugin, adding a model will make all existing instances of a plugin disappear from the editor.
  
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  ## Steps to reproduce
  
   `
      
          @plugin_pool.register_plugin
          class ColumnPlugin(CMSPluginBase):
              name = _("Column")
              # model = models.Column    <-- adding model to plugin that already exists
              render_template = "cmsplugin_column/cmsplugin_column.html"
              allow_children = True
              require_parent = True
              parent_classes = ["RowPlugin"]
          
              def render(self, context, instance, placeholder):
                  context = super().render(context, instance, placeholder)
                  return context
  
  `
  
  1. create the plugin above, initially without a model
  2. create a couple instances of the plugin on the page
  3. add the model to the plugin ( uncomment the commented line above )
  4. plugins will disappear from the page
  
  ## Expected behaviour
  
  I would expect that either plugins show up, but without any settings, or that some error displays, indicating that some plugins dont have corresponding model isntances.
  
  ## Actual behaviour
  
  Plugins disappear without any warning or indication
  
  
  ## Additional information (CMS/Python/Django versions)
  
  django==3.2.14
  django-cms==3.11.0
  
  
  ## Do you want to help fix this issue?
  
  Could this be solved via an manage.py command that creates default model instances for plugins that hove none? I could help creating a manage.py command if I had some guidance how to correctly query for this case.
  
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  * [ ] Yes, I want to help fix this issue and I will join #workgroup-pr-review on [Slack](https://www.django-cms.org/slack) to confirm with the community that a PR is welcome. 
  * [x ] No, I only want to report the issue.
  