Fixed VueJS warning for border top

app/views/projects/boards/components/_board.html.haml

 %board{ "inline-template" => true, "v-cloak" => true, "v-for" => "board in state.lists | orderBy 'position'", ":board" => "board" }
   .board{ ":class" => "{ 'is-draggable': !isPreset }" }
     .board-inner
-      %header.board-inner-container.board-header{ ":class" => "{ 'has-border': board.label }", ":style" => "{ borderTopColor: board.label.color }" }
+      %header.board-inner-container.board-header{ ":class" => "{ 'has-border': board.label }", ":style" => "{ borderTopColor: (board.label ? board.label.color : null) }" }
         %h3.board-title.js-board-handle
           {{ board.title }}
           %span.pull-right{ "v-if" => "board.type !== 'blank'" }