How to save that a Button was pressed that he was pressed

I currently made up my slot based saving system in which i can save all variables except for one. My Question is, how can I save my skilltree. I have a pretty basic setup, as soon as I get the first skill the second one gets activated and as soon as this skill is skilled the third one is activated. How can I save this? If I open my Skill tree it should load and if i close it it should be saved (i set it to collapsed). Currently it only works without Saving. I can skill every skill once and that is it but if i try to save my entire skilltree it just restarts the skilltree.
To sum it up, how can i save everything the button does and if i load my save state it will stay saved?

There is no straightforward way to do that, it all depends on your set-up and how you “unlock” your skill tree. The way I tackled this problem in my game for opening up “levels” once the player has beaten the previous level was to store all the level “buttons” (as my levels were clickable icons on a map) into an array. and hide them. I then had an actor in each level that identified the level being played by an integer value. So for example, this “master” actor had an integer variable in it that was editable per instance. Each level I created I placed this actor into it and set the integer variable to match the level so, level 1 had this variable set to 1, 2 was set to 2 and so on. When it came time to “save” my game, I pulled this integer value from the “master” actor and stored it away. When I re-load the game I would grab this stored integer and use it to loop through my array of “level buttons” setting each level button to visible (default was hidden) up to the integer value I had stored that corresponded to the level the player was last on. This opened all levels that were less than or equal to the saved integer value and thus “saved” the last unlocked level. You have to figure out a way that is logical for your set-up, it is not like there is a simple “save current position in skill tree” node. Anyway, hope that helps to at least point you in the right direction.