[gtkmm-documentation] Don't crash when showing GtkCssProvider errors.

[Murray Cumming <murrayc src gnome org>]

commit cd14745f0a40c96aa2582a44eee742caa1719b22
Author: Murray Cumming <murrayc murrayc com>
Date:   Mon May 16 19:54:35 2016 +0200

    Don't crash when showing GtkCssProvider errors.

 examples/book/custom/custom_widget/mywidget.cc |    7 ++++++-
 examples/others/calendar/calendar.cc           |    7 ++++++-
 2 files changed, 12 insertions(+), 2 deletions(-)
---
diff --git a/examples/book/custom/custom_widget/mywidget.cc b/examples/book/custom/custom_widget/mywidget.cc
index 39ceac4..b64021f 100644
--- a/examples/book/custom/custom_widget/mywidget.cc
+++ b/examples/book/custom/custom_widget/mywidget.cc
@@ -238,7 +238,12 @@ void MyWidget::on_parsing_error(const Glib::RefPtr<const Gtk::CssSection>& secti
   std::cerr << "on_parsing_error(): " << error.what() << std::endl;
   if (section)
   {
-    std::cerr << "  URI = " << section->get_file()->get_uri() << std::endl;
+    const auto file = section->get_file();
+    if (file)
+    {
+      std::cerr << "  URI = " << file->get_uri() << std::endl;
+    }
+
     std::cerr << "  start_line = " << section->get_start_line()+1
               << ", end_line = " << section->get_end_line()+1 << std::endl;
     std::cerr << "  start_position = " << section->get_start_position()
diff --git a/examples/others/calendar/calendar.cc b/examples/others/calendar/calendar.cc
index b93425b..4e95114 100644
--- a/examples/others/calendar/calendar.cc
+++ b/examples/others/calendar/calendar.cc
@@ -148,7 +148,12 @@ void CalendarExample::on_parsing_error(const Glib::RefPtr<const Gtk::CssSection>
   std::cerr << "on_parsing_error(): " << error.what() << std::endl;
   if (section)
   {
-    std::cerr << "  URI = " << section->get_file()->get_uri() << std::endl;
+    const auto file = section->get_file();
+    if (file)
+    {
+      std::cerr << "  URI = " << file->get_uri() << std::endl;
+    }
+
     std::cerr << "  start_line = " << section->get_start_line()+1
               << ", end_line = " << section->get_end_line()+1 << std::endl;
     std::cerr << "  start_position = " << section->get_start_position()