Description of problem:
when pitivi is started from the command line, it runs, but does output a non-fatal error message:
(pitivi:2572): Gtk-WARNING **: Allocating size to GtkScrollbar 0x55f282a7cee0 without calling gtk_widget_get_preferred_width/height(). How does the code know the size to allocate?

Version-Release number of selected component (if applicable):
$ rpm -qil pitivi
Name        : pitivi
Version     : 0.98
Release     : 7.fc26
Architecture: x86_64
Install Date: Mon 12 Jun 2017 08:48:42 PM
Group       : Unspecified
Size        : 9901692
License     : LGPLv2+
Signature   : RSA/SHA256, Sat 10 Jun 2017 06:02:10 AM, Key ID 812a6b4b64dab85d
Source RPM  : pitivi-0.98-7.fc26.src.rpm
Build Date  : Fri 09 Jun 2017 11:11:01 PM
Build Host  : buildvm-23.phx2.fedoraproject.org
Relocations : (not relocatable)
Packager    : Fedora Project
Vendor      : Fedora Project
URL         : http://www.pitivi.org/

How reproducible:
Fully updated Fedora 26 beta and start pitivi from the commandline.

Steps to Reproduce:
1. open terminal
2. type in pitivi
3. pitivi starts but also prints out the error message about resizing.

Actual results:

Pitivi runs fine which is good, but since it does print out that error message, it might be useful to fix it.

Expected results:

not to see the error message.

Additional info:

none.