Description of problem:
Steps:
- Right-clicked on Menu > Configure...
- Opened file chooser to select custom icon
- Navigated to an icon in /usr/share/icons and clicked on Open button

Version-Release number of selected component:
cinnamon-3.2.1-1.fc25

Additional info:
reporter:       libreport-2.8.0
cmdline:        python3 /usr/share/cinnamon/cinnamon-settings/xlet-settings.py applet menu 0
environ:        
executable:     /usr/share/cinnamon/cinnamon-settings/xlet-settings.py
kernel:         4.8.8-300.fc25.x86_64
pkg_fingerprint: 4089 D8F2 FDB1 9C98
pkg_vendor:     Fedora Project
runlevel:       N 5
type:           Python3
uid:            1000

Truncated backtrace:
SettingsWidgets.py:984:on_button_pressed:AttributeError: 'NewClass' object has no attribute 'set_val'

Traceback (most recent call last):
  File "/usr/share/cinnamon/cinnamon-settings/bin/SettingsWidgets.py", line 984, in on_button_pressed
    self.set_val(filename)
AttributeError: 'NewClass' object has no attribute 'set_val'

Local variables in innermost frame:
dialog: <Gtk.FileChooserDialog object at 0x7f988eff2798 (GtkFileChooserDialog at 0x55bd514ce340)>
self: <JsonSettingsWidgets.NewClass object at 0x7f988efe4e10 (JsonSettingsWidgets+NewClass-v11 at 0x55bd5119bf50)>
response: -5
preview: <Gtk.Image object at 0x7f988eff2990 (GtkImage at 0x55bd515f9420)>
filter_text: <Gtk.FileFilter object at 0x7f988eff2948 (GtkFileFilter at 0x55bd515fd980)>
widget: <Gtk.Button object at 0x7f988efe6c18 (GtkButton at 0x55bd51203500)>
filename: '/usr/share/icons/Numix-Circle/scalable/apps/fedora-utils.svg'