glib: fix leak

Forgot to free FormPageWidgets* object
returned by Page::getFormWidgets()
7 jobs for glib_memleak in 6 minutes and 54 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Build
passed #7861903
build

00:06:33

passed #7861908
build_android

00:01:00

passed #7861904
build_clang_libcpp

00:06:54

passed #7861907
build_clazy

00:03:36

passed #7861906
build_mingw64_fedora

00:03:18

passed #7861905
build_ubuntu_18_04

00:03:20

passed #7861902
clang_format

00:01:20