Can't Replace Matrix Fieldset #245

Describe the issue

500 error is thrown when trying to change the 'Fieldset' value of an existing matrix from one fieldset to another.

message: "Trying to get property 'fields' of non-object"
exception: "ErrorException"
file: "/home/vagrant/Code/Efelle/fusioncms/app/Concerns/HasFieldset.php"
line: 40

Only occurs when replacing an existing value, and not when adding a fieldset when none was present before or when removing a fieldset.

To reproduce

Create a new fieldset, and then create two fieldsets. Attach the first fieldset to the matrix, and save. Then, edit the matrix and change the fieldset to the second fieldset created.

Expected behavior

The fieldset should correctly be replaced with no errors.

Bug report checklist

• I have filled out as much information as I can
• I have detailed how the issue can be reproduced
• I have searched for existing issues and to the best of my knowledge this is not a duplicate