title: How to fix "isinstance() arg 2 must be a type or tuple of types" error in Django
body: <p>I created a Django  (v. 2.1.5) model called Metric that has itself as an embed model, as you can see below:</p>
  
  <pre class="lang-py prettyprint-override"><code>from djongo import models
  
  class Metric(models.Model):
      _id = models.ObjectIdField()
     ...
      dependencies = models.ArrayModelField(
          model_container='Metric',
          blank=True,
      )
  
      def __str__(self):
          return self.name
  
      class Meta:
          db_table = 'metric'
  </code></pre>
  
  <p>But, when I try to execute the code:</p>
  
  <pre class="lang-py prettyprint-override"><code>for metric in Metric.objects.all():
  </code></pre>
  
  <p>I get the following error:</p>
  
  <pre class="lang-py prettyprint-override"><code>  File "/.../python3.6/site-packages/djongo/models/fields.py", line 235, in to_python
      if isinstance(mdl_dict, self.model_container):
  TypeError: isinstance() arg 2 must be a type or tuple of types
  </code></pre>
  
  <p>I imagine that this error was caused by the use of single quotation marks on model_container assignment, but I can't remove it, since the model_container is the class itself. Also, I'm not sure if that is the reason.</p>
  
  <p>In any case, what I can do to fix this error?</p>