pull-requests: fixed cases with default expected refs are closed or unavailable.

- for Mercurial with closed default branch a compare across forks can fail.
- fixes  #5436

rhodecode/public/js/src/rhodecode/pullrequests.js

@@ -217,6 +217,12 b' ReviewersController = function () {'
         prButtonLock(true, null, 'reviewers');
         $('#user').hide(); // hide user autocomplete before load
 
+        if (sourceRef.length !== 3 || targetRef.length !== 3) {
+            // don't load defaults in case we're missing some refs...
+            $('.calculate-reviewers').hide();
+            return
+        }
+
         var url = pyroutes.url('repo_default_reviewers_data',
                 {
                     'repo_name': templateContext.repo_name,

rhodecode/templates/pullrequests/pullrequest.mako

@@ -202,9 +202,13 b''
           // translate our select2 id into a text, it's a mapping to show
           // simple label when selecting by internal ID.
           var id, refData;
-          if (selectedRef === undefined) {
+          if (selectedRef === undefined || selectedRef === null) {
             id = element.val();
             refData = element.val().split(':');
+
+            if (refData.length !== 3){
+                refData = ["", "", ""]
+            }
           } else {
             id = selectedRef;
             refData = selectedRef.split(':');
@@ -216,7 +220,6 b''
           }
 
           var data = {id: id, text: text};
-
           callback(data);
       };
   };
@@ -415,8 +418,10 b''
     };
   };
 
-  var initTargetRefs = function(refsData, selectedRef){
+  var initTargetRefs = function(refsData, selectedRef) {
+
     Select2Box($targetRef, {
+        placeholder: "${_('Select commit reference')}",
       query: function(query) {
         queryTargetRefs(refsData, query);
       },
@@ -518,7 +523,9 b''
   % if c.default_source_ref:
   // in case we have a pre-selected value, use it now
   $sourceRef.select2('val', '${c.default_source_ref}');
+  // diff preview load
   loadRepoRefDiffPreview();
+  // default reviewers
   reviewersController.loadDefaultReviewers(
       sourceRepo(), sourceRef(), targetRepo(), targetRef());
   % endif