Java exception occurred: java.lang.​IllegalSta​teExceptio​n: node does not allow children

Hello,

I am trying to create a node tree. In the script, I created the root of the tree and added several nodes to it. However when I use the recursive function: this function will create more nodes for the tree itself, I am receiving this error

Java exception occurred:

java.lang.IllegalStateException: node does not allow children

at javax.swing.tree.DefaultMutableTreeNode.insert(DefaultMutableTreeNode.java:175)

at javax.swing.tree.DefaultMutableTreeNode.add(DefaultMutableTreeNode.java:411)

Does anyone know what it means?