Up/Down keys are not secured against no children

[jmesa-sistel]

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Current behavior

Expected behavior

The problem is when you expand a node with no children.
In this case down/up keys try to access to children and you get an error in console

Minimal reproduction of the problem with instructions

Create a tree with an expandable node that has no children.
Expand that node, now try to move up or down, you will get an error in console

• PrimeNG version: 6.1.5

The fix is protect the code.
In onKeyDown function:

 case 40:
        var listElement = nodeElement.children[0].children[1];
        if (listElement && listElement.children.length > 0) {      // <-- Protect against no children
             this.focusNode(listElement.children[0]);
        }

In findLastVisibleDescendant function:

        var childrenListElement = nodeElement.children[0].children[1];
        if (childrenListElement && childrenListElement.children.length > 0) {  // <-- Protect against no children
            var lastChildElement = childrenListElement.children[childrenListElement.children.length - 1];
            return this.findLastVisibleDescendant(lastChildElement);
        }
        else {
            return nodeElement;
        }

[jmesa-sistel]

@cagataycivici
Any news on this bug?
The fix is very simple.

[jmesa-sistel]

@mertsincan Can you check it?