Upgrading to version 6.2.1 causes "KeyError FormPlugin"

We bumped the aldryn-forms version from 4.0.1 to 6.2.1 and got the error below. Version 5.0.4 works fine.
KeyError: 'FormPlugin'
  File "django/core/handlers/exception.py", line 34, in inner
    response = get_response(request)
  File "django/core/handlers/base.py", line 145, in _get_response
    response = self.process_exception_by_middleware(e, request)
  File "django/core/handlers/base.py", line 143, in _get_response
    response = response.render()
  File "django/template/response.py", line 106, in render
    self.content = self.rendered_content
  File "django/template/response.py", line 83, in rendered_content
    content = template.render(context, self._request)
  File "django/template/backends/django.py", line 61, in render
    return self.template.render(context)
  File "django/template/base.py", line 171, in render
    return self._render(context)
  File "django/template/base.py", line 163, in _render
    return self.nodelist.render(context)
  File "django/template/base.py", line 937, in render
    bit = node.render_annotated(context)
  File "django/template/base.py", line 904, in render_annotated
    return self.render(context)
  File "django/template/loader_tags.py", line 150, in render
    return compiled_parent._render(context)
  File "django/template/base.py", line 163, in _render
    return self.nodelist.render(context)
  File "django/template/base.py", line 937, in render
    bit = node.render_annotated(context)
  File "django/template/base.py", line 904, in render_annotated
    return self.render(context)
  File "classytags/core.py", line 151, in render
    return self.render_tag(context, **kwargs)
  File "sekizai/templatetags/sekizai_tags.py", line 87, in render_tag
    rendered_contents = nodelist.render(context)
  File "django/template/base.py", line 937, in render
    bit = node.render_annotated(context)
  File "django/template/base.py", line 904, in render_annotated
    return self.render(context)
  File "classytags/core.py", line 151, in render
    return self.render_tag(context, **kwargs)
  File "cms/templatetags/cms_tags.py", line 447, in render_tag
    return toolbar.render_with_structure(context, nodelist)
  File "cms/toolbar/toolbar.py", line 477, in render_with_structure
    rendered_contents = nodelist.render(context)
  File "django/template/base.py", line 937, in render
    bit = node.render_annotated(context)
  File "django/template/base.py", line 904, in render_annotated
    return self.render(context)
  File "django/template/loader_tags.py", line 62, in render
    result = block.nodelist.render(context)
  File "django/template/base.py", line 937, in render
    bit = node.render_annotated(context)
  File "django/template/base.py", line 904, in render_annotated
    return self.render(context)
  File "classytags/core.py", line 151, in render
    return self.render_tag(context, **kwargs)
  File "cms/templatetags/cms_tags.py", line 313, in render_tag
    nodelist=nodelist,
  File "cms/plugin_rendering.py", line 329, in render_page_placeholder
    self._preload_placeholders_for_page(current_page)
  File "cms/plugin_rendering.py", line 529, in _preload_placeholders_for_page
    is_fallback=inherit,
  File "cms/utils/plugins.py", line 75, in assign_plugins
    plugin_groups = dict((key, list(plugins)) for key, plugins in groupby(plugins, attrgetter('placeholder_id')))
  File "cms/utils/plugins.py", line 75, in <genexpr>
    plugin_groups = dict((key, list(plugins)) for key, plugins in groupby(plugins, attrgetter('placeholder_id')))
  File "cms/utils/plugins.py", line 275, in downcast_plugins
    cls = plugin_pool.get_plugin(plugin_type)
  File "cms/plugin_pool.py", line 189, in get_plugin
    return self.plugins[name]
divio / aldryn-forms

Upgrading to version 6.2.1 causes "KeyError FormPlugin" #221
