[glom] Add null checks to silence clang-tidy.

[Murray Cumming <murrayc src gnome org>]

commit 5a0a122a78cf732e0a0413958a6d444f804c218b
Author: Murray Cumming <murrayc murrayc com>
Date:   Wed Feb 10 19:12:54 2016 +0100

    Add null checks to silence clang-tidy.

 glom/frame_glom.cc                                 |    9 ++++++++-
 glom/mode_data/datawidget/cellrenderer_dblist.cc   |    2 ++
 .../report_layout/dialog_layout_report.cc          |    6 ++++++
 3 files changed, 16 insertions(+), 1 deletions(-)
---
diff --git a/glom/frame_glom.cc b/glom/frame_glom.cc
index 58d3bd8..8de6b5c 100644
--- a/glom/frame_glom.cc
+++ b/glom/frame_glom.cc
@@ -1565,6 +1565,11 @@ void Frame_Glom::do_menu_developer_fields(Gtk::Window& parent, const Glib::ustri
   if(!m_pDialog_Fields)
   {
     Utils::get_glade_widget_derived_with_warning(m_pDialog_Fields);
+    if (!m_pDialog_Fields) {
+      std::cerr << G_STRFUNC << ": m_pDialog_Fields is null." << std::endl;
+      return;
+    }
+
     m_pDialog_Fields->signal_hide().connect( sigc::mem_fun(*this, &Frame_Glom::on_developer_dialog_hide));
     add_view(m_pDialog_Fields);
   }
@@ -1987,7 +1992,9 @@ void Frame_Glom::instantiate_dialog_connection()
 
   add_view(m_pDialogConnection); //Also a composite view.
 
-  m_pDialogConnection->set_transient_for(*get_app_window());
+  auto window = get_app_window();
+  if (window)
+    m_pDialogConnection->set_transient_for(*window);
 }
 
 bool Frame_Glom::connection_request_password_and_choose_new_database_name()
diff --git a/glom/mode_data/datawidget/cellrenderer_dblist.cc 
b/glom/mode_data/datawidget/cellrenderer_dblist.cc
index 16151e6..10107e6 100644
--- a/glom/mode_data/datawidget/cellrenderer_dblist.cc
+++ b/glom/mode_data/datawidget/cellrenderer_dblist.cc
@@ -92,6 +92,7 @@ void CellRendererDbList::repack_cells_fixed(Gtk::CellLayout* combobox)
   if(!widget)
   {
     std::cerr << G_STRFUNC << ": widget is null.\n";
+    return;
   }
 
   if(!m_repacked_first_cell)
@@ -152,6 +153,7 @@ void CellRendererDbList::repack_cells_related(Gtk::CellLayout* combobox)
   if(!widget)
   {
     std::cerr << G_STRFUNC << ": widget is null.\n";
+    return;
   }
 
   const auto cells = combobox->get_cells();
diff --git a/glom/mode_design/report_layout/dialog_layout_report.cc 
b/glom/mode_design/report_layout/dialog_layout_report.cc
index aec02b5..2258541 100644
--- a/glom/mode_design/report_layout/dialog_layout_report.cc
+++ b/glom/mode_design/report_layout/dialog_layout_report.cc
@@ -824,6 +824,12 @@ void Dialog_Layout_Report::on_button_edit()
       {
         Dialog_FieldSummary* dialog = nullptr;
         Utils::get_glade_widget_derived_with_warning(dialog);
+        if(!dialog)
+        {
+          std::cerr << G_STRFUNC << ": dialog is null." << std::endl;
+          return;
+        }
+
         add_view(dialog);
         dialog->set_item(fieldsummary, m_table_name);
         dialog->set_transient_for(*this);