Merge pull request gotk3#931 from mintsoft/fix_callback

Fixing undefined: callback

gdk/gdk_since_3_22.go

@@ -26,6 +26,7 @@ import (
 	"unsafe"
 
 	"github.com/gotk3/gotk3/glib"
+	"github.com/gotk3/gotk3/internal/callback"
 )
 
 func init() {
@@ -137,7 +138,12 @@ func (v *Seat) GetDisplay() (*Display, error) {
 
 // Grab is a wrapper around gdk_seat_grab().
 func (v *Seat) Grab(window *Window, capabilities SeatCapabilities, owner_events bool, cursor *Cursor, event *Event, prepare_func GrabPrepareFunc, prepare_func_data C.gpointer) GrabStatus {
-    return GrabStatus(C.gdk_seat_grab(v.native(), window.native(), C.GdkSeatCapabilities(capabilities), gbool(owner_events), cursor.native(), event.native(), (*[0]byte)(C.gpointer(callback.Assign(prepare_func))), prepare_func_data))
+    prepare_func_wrapped := (*[0]byte)(nil)
+    if(prepare_func != nil) {
+        prepare_func_wrapped = (*[0]byte)(C.gpointer(callback.Assign(prepare_func)))
+    }
+
+    return GrabStatus(C.gdk_seat_grab(v.native(), window.native(), C.GdkSeatCapabilities(capabilities), gbool(owner_events), cursor.native(), event.native(), prepare_func_wrapped, prepare_func_data))
 }
 
 // UnGrab is a wrapper around gdk_seat_ungrab().