The alternate solution is to pass the parameter for the custom form via the URL.
1. Navigate Customization > Forms > Transaction Forms.
2. Look for the Standard Estimate Form ID (Example 97).
3. Navigate back to the Estimate record that is giving an error.
4. On the URL add the parametercf=97& after the .nl? where 97 is the standard estimate form.
To get the form ID, navigate to Customization > Forms > Transaction Forms and open the desired form and look at the value after id= in the URL.