Fix crashes when e_cal_util_get_system_timezone() returns NULL

The return value is nullable, but gnome-calendar fails to check it.

Because the nullable return value is inconvenient for us, we can instead
add and use gcal_context_get_icaltimezone() to supplement the existing
gcal_context_get_timezone().

We'll also need a function to convert from GTimeZone to ICalTimezone,
gcal_timezone_to_icaltimezone().

Fixes #419

src/core/gcal-event.c

@@ -20,6 +20,7 @@
 
 #include "gconstructor.h"
 #include "gcal-application.h"
+#include "gcal-context.h"
 #include "gcal-debug.h"
 #include "gcal-event.h"
 #include "gcal-utils.h"
@@ -223,12 +224,17 @@ static ECalComponentDateTime*
 build_component_from_datetime (GcalEvent *self,
                                GDateTime *dt)
 {
+  GcalApplication *application;
+  GcalContext *context;
   ICalTime *itt;
   gchar *tzid;
 
   if (!dt)
     return NULL;
 
+  application = GCAL_APPLICATION (g_application_get_default ());
+  context = gcal_application_get_context (application);
+
   itt = gcal_date_time_to_icaltime (dt);
 
   if (self->all_day)
@@ -238,11 +244,13 @@ build_component_from_datetime (GcalEvent *self,
     }
   else
     {
-      ICalTimezone *zone;
+      ICalTimezone *tz;
+      GTimeZone *zone;
 
-      zone = e_cal_util_get_system_timezone ();
-      i_cal_time_set_timezone (itt, zone);
-      tzid = zone ? g_strdup (i_cal_timezone_get_tzid (zone)) : NULL;
+      zone = gcal_context_get_timezone (context);
+      tz = gcal_timezone_to_icaltimezone (zone);
+      i_cal_time_set_timezone (itt, tz);
+      tzid = g_strdup (i_cal_timezone_get_tzid (tz));
     }
 
   /* Call it after setting the timezone, because the DATE values do not let set the timezone */

src/core/gcal-manager.c

@@ -18,6 +18,8 @@
 
 #define G_LOG_DOMAIN "GcalManager"
 
+#include "gcal-application.h"
+#include "gcal-context.h"
 #include "gcal-debug.h"
 #include "gcal-manager.h"
 #include "gcal-utils.h"
@@ -877,11 +879,18 @@ void
 gcal_manager_setup_shell_search (GcalManager             *self,
                                  ECalDataModelSubscriber *subscriber)
 {
+  GcalApplication *application;
+  GcalContext *context;
+  GTimeZone *zone;
+
   g_return_if_fail (GCAL_IS_MANAGER (self));
 
   if (self->shell_search_data_model)
     return;
 
+  application = GCAL_APPLICATION (g_application_get_default ());
+  context = gcal_application_get_context (application);
+
   self->shell_search_data_model = e_cal_data_model_new (gcal_thread_submit_job);
   g_signal_connect_object (self->shell_search_data_model,
                            "view-state-changed",
@@ -890,7 +899,9 @@ gcal_manager_setup_shell_search (GcalManager             *self,
                            G_CONNECT_SWAPPED);
 
   e_cal_data_model_set_expand_recurrences (self->shell_search_data_model, TRUE);
-  e_cal_data_model_set_timezone (self->shell_search_data_model, e_cal_util_get_system_timezone ());
+
+  zone = gcal_context_get_timezone (context);
+  e_cal_data_model_set_timezone (self->shell_search_data_model, gcal_timezone_to_icaltimezone (zone));
 
   self->search_view_data = g_new0 (ViewStateData, 1);
   self->search_view_data->subscriber = subscriber;
@@ -1420,7 +1431,11 @@ gcal_manager_get_events (GcalManager *self,
                          ICalTime    *start_date,
                          ICalTime    *end_date)
 {
+  GcalApplication *application;
+  GcalContext *context;
   time_t range_start, range_end;
+  GTimeZone *zone;
+  ICalTimezone *tz;
   GatherEventData data = {
     .manager = self,
     .events = NULL,
@@ -1428,10 +1443,15 @@ gcal_manager_get_events (GcalManager *self,
 
   GCAL_ENTRY;
 
+  application = GCAL_APPLICATION (g_application_get_default ());
+  context = gcal_application_get_context (application);
+
   g_return_val_if_fail (GCAL_IS_MANAGER (self), NULL);
 
-  range_start = i_cal_time_as_timet_with_zone (start_date, e_cal_util_get_system_timezone ());
-  range_end = i_cal_time_as_timet_with_zone (end_date, e_cal_util_get_system_timezone ());
+  zone = gcal_context_get_timezone (context);
+  tz = gcal_timezone_to_icaltimezone (zone);
+  range_start = i_cal_time_as_timet_with_zone (start_date, tz);
+  range_end = i_cal_time_as_timet_with_zone (end_date, tz);
 
   e_cal_data_model_foreach_component (self->e_data_model,
                                       range_start,
@@ -1521,9 +1541,15 @@ gcal_manager_startup (GcalManager *self)
   GList *sources, *l;
   GError *error = NULL;
   ESourceCredentialsProvider *credentials_provider;
+  GcalApplication *application;
+  GcalContext *context;
+  GTimeZone *zone;
 
   GCAL_ENTRY;
 
+  application = GCAL_APPLICATION (g_application_get_default ());
+  context = gcal_application_get_context (application);
+
   self->clients = g_hash_table_new_full ((GHashFunc) e_source_hash,
                                          (GEqualFunc) e_source_equal,
                                          g_object_unref,
@@ -1621,7 +1647,9 @@ gcal_manager_startup (GcalManager *self)
   self->e_data_model = e_cal_data_model_new (gcal_thread_submit_job);
 
   e_cal_data_model_set_expand_recurrences (self->e_data_model, TRUE);
-  e_cal_data_model_set_timezone (self->e_data_model, e_cal_util_get_system_timezone ());
+
+  zone = gcal_context_get_timezone (context);
+  e_cal_data_model_set_timezone (self->e_data_model, gcal_timezone_to_icaltimezone (zone));
 
   sources = e_source_registry_list_enabled (self->source_registry, E_SOURCE_EXTENSION_CALENDAR);