Behavior Tree is disabled

I have created a BP using Behavior Tree, and accidentally i closed the UE editor, when i try to open the same BP. I am not able to swtich to Behavior tree, it is showing disabled. How can i open my behavior tree back ??

I tried searching over internet , not able to find any solution. i got a solution at Behavior tree inactive - Blueprint - Unreal Engine Forums , but it saying to create new. I want to open my previous tree. Please help