title: Using SimpleForm inside DataGrid and ReferenceManyField | react-admin
body: <p>actually i try to use a SimpleForm inside a Datagrid and ReferenceManyField to show a joined table and also change this data. Showing the data and deleting works pretty fine but everytime i click on Save i get the following Error:</p>
  
  <p>`</p>
  
  <pre><code>Uncaught TypeError: onSave.current is not a function
      at submit (FormWithRedirect.js:86)
      at Object.submit (final-form.es.js:1296)
      at handleSubmit (react-final-form.es.js:264)
      at FormWithRedirect.js:121
      at handleClick (SaveButton.js:57)
  </code></pre>
  
  <p>`</p>
  
  <p>My actual Code:</p>
  
  <pre><code>&lt;Edit {...props}&gt;
          &lt;SimpleShowLayout&gt;
              &lt;DateField source="date" /&gt;
              &lt;ReferenceField label="Location" source="LocationID" reference="locations"&gt;
                  &lt;TextField source="name" /&gt;
              &lt;/ReferenceField&gt;
              &lt;ReferenceField label="Employee" source="EmployeeID" reference="employees"&gt;
                  &lt;TextField source="first_name" /&gt;
              &lt;/ReferenceField&gt;
              &lt;ReferenceManyField
                  label="Customers"
                  reference="bookings"
                  target="data"
              &gt;
                  &lt;Datagrid&gt;
                      &lt;SimpleForm&gt;
                          &lt;TextInput label="Vorname" source="customer.first_name" /&gt;
                          &lt;TextInput label="Nachname" source="customer.last_name" /&gt;
  
                          &lt;BooleanInput label="Anwesend" source="present" /&gt;
                      &lt;/SimpleForm&gt;
                  &lt;/Datagrid&gt;
              &lt;/ReferenceManyField&gt;
          &lt;/SimpleShowLayout&gt;
      &lt;/Edit&gt;
  </code></pre>