UI: Drag and Drop Constraints, Layout Updates

This patch implements the list panel system D7490 for constraints.
In this case the panels are still defined in Python.

The layouts are also updated to use subpanels and the a more organized
single column layout. There may be more tweaks necessary for the
layouts.

Reviewed By: Severin, billreynish, Mets

Differential Revision: https://developer.blender.org/D7499

1 files changed, 12 insertions, 2 deletions

diff --git a/source/blender/editors/interface/interface_panel.c b/source/blender/editors/interface/interface_panel.c
index 4fff80d5def..924b886f167 100644
--- a/source/blender/editors/interface/interface_panel.c
+++ b/source/blender/editors/interface/interface_panel.c
@@ -387,9 +387,19 @@ bool UI_panel_list_matches_data(ARegion *region,
                                 ListBase *data,
                                 uiListPanelIDFromDataFunc panel_idname_func)
 {
-  int data_len = BLI_listbase_count(data);
+  /* Check for NULL data. */
+  int data_len = 0;
+  Link *data_link = NULL;
+  if (data == NULL) {
+    data_len = 0;
+    data_link = NULL;
+  }
+  else {
+    data_len = BLI_listbase_count(data);
+    data_link = data->first;
+  }
+
   int i = 0;
-  Link *data_link = data->first;
   LISTBASE_FOREACH (Panel *, panel, &region->panels) {
     if (panel->type != NULL && panel->type->flag & PNL_INSTANCED) {
       /* The panels were reordered by drag and drop. */