Manually merged fixes from seve12.

Thanks to seve12 !

Related https://github.com/wekan/wekan/pull/5967

client/components/lists/listBody.jade

@@ -85,16 +85,19 @@ template(name="linkCardPopup")
 
   label {{_ 'swimlanes'}}:
   select.js-select-swimlanes
+    option(value="") {{_ 'custom-field-dropdown-none'}}
     each swimlanes
       option(value="{{_id}}") {{isTitleDefault title}}
 
   label {{_ 'lists'}}:
   select.js-select-lists
+    option(value="") {{_ 'custom-field-dropdown-none'}}
     each lists
       option(value="{{_id}}") {{isTitleDefault title}}
 
   label {{_ 'cards'}}:
   select.js-select-cards
+    option(value="") {{_ 'custom-field-dropdown-none'}}
     each cards
       option(value="{{getRealId}}") {{getTitle}}
 

client/components/lists/listBody.js

@@ -542,8 +542,6 @@ BlazeComponent.extendComponent({
     {
       sort: { sort: 1 },
     });
-    if (swimlanes.length)
-      this.selectedSwimlaneId.set(swimlanes[0]._id);
     return swimlanes;
   },
 
@@ -558,7 +556,6 @@ BlazeComponent.extendComponent({
     {
       sort: { sort: 1 },
     });
-    if (lists.length) this.selectedListId.set(lists[0]._id);
     return lists;
   },
 
@@ -567,19 +564,17 @@ BlazeComponent.extendComponent({
       return [];
     }
     const ownCardsIds = this.board.cards().map(card => card.getRealId());
-    const ret = ReactiveCache.getCards(
-    {
-      boardId: this.selectedBoardId.get(),
-      swimlaneId: this.selectedSwimlaneId.get(),
-      listId: this.selectedListId.get(),
+    const selector = {
       archived: false,
       linkedId: { $nin: ownCardsIds },
       _id: { $nin: ownCardsIds },
       type: { $nin: ['template-card'] },
-    },
-    {
-      sort: { sort: 1 },
-    });
+    };
+    if (this.selectedBoardId.get()) selector.boardId = this.selectedBoardId.get();
+    if (this.selectedSwimlaneId.get()) selector.swimlaneId = this.selectedSwimlaneId.get();
+    if (this.selectedListId.get()) selector.listId = this.selectedListId.get();
+
+    const ret = ReactiveCache.getCards(selector, { sort: { sort: 1 } });
     return ret;
   },
 
@@ -600,8 +595,12 @@ BlazeComponent.extendComponent({
     return [
       {
         'change .js-select-boards'(evt) {
-          subManager.subscribe('board', $(evt.currentTarget).val(), false);
-          this.selectedBoardId.set($(evt.currentTarget).val());
+          const val = $(evt.currentTarget).val();
+          subManager.subscribe('board', val, false);
+          // Clear selections to allow linking only board or re-choose swimlane/list
+          this.selectedSwimlaneId.set('');
+          this.selectedListId.set('');
+          this.selectedBoardId.set(val);
         },
         'change .js-select-swimlanes'(evt) {
           this.selectedSwimlaneId.set($(evt.currentTarget).val());