Hi Yi Wan,
"Warning: Functionality not supported with figures created with the uifigure function. For more information, see Graphics Support in App Designer. "
It is not the main reason to cause your code not working. It is a warning to remind instead of error.
First, your uisave should be working fine and able to create the mat.file for you.
Second, It is succesful to load the file as your expected, however, your code below is not working.
Third, hence, I recommend you to add a new delete button to clear to content of the tree first, then later load the mat.file into the tree
function d(app, event)
Fourth, now we load the mat. file and get the content add into the tree by following method
function LoadProgramButtonPushed(app, event)
category = t.Children;
tree1(i) = uitreenode(app.Tree);
tree1(i).Text = category(i).Text;
content = category(i).Children;
for j =1:1:length(content)
tree2(i,j) = uitreenode(tree1(i));
tree2(i,j).Text = content(j).Text;
Now, i guess it is working well with your case.