Return null instead of throwing of Widget.child does not exist

src/dlangui/core/collections.d

@@ -248,6 +248,17 @@ struct ObjectList(T) {
         assert(index >= 0 && index < _count, "child index out of range");
         return _list[index];
     }
+    /// get item by index. Returns null if item not found
+    inout(T) tryGet(int index) @safe inout nothrow {
+        try {
+            if (index < 0 || index >= _count) {
+                return null;
+            }
+            return _list[index];
+        } catch (Exception e) {
+            return null;
+        }
+    }
     /// get item by index
     T opIndex(int index) @safe {
         return get(index);

src/dlangui/widgets/tree.d

@@ -279,7 +279,7 @@ class TreeItem {
     /// returns number of children of this widget
     @property int childCount() { return _children.count; }
     /// returns child by index
-    TreeItem child(int index) { return _children.get(index); }
+    TreeItem child(int index) { return _children.tryGet(index); }
     /// adds child, returns added item
     TreeItem addChild(TreeItem item, int index = -1) {
         TreeItem res = _children.insert(item, index).parent(this).level(_level + 1);

src/dlangui/widgets/widget.d

@@ -1821,7 +1821,7 @@ class WidgetGroup : Widget {
     /// returns number of children of this widget
     @property override int childCount() const { return _children.count; }
     /// returns child by index
-    override inout(Widget) child(int index) inout { return _children.get(index); }
+    override inout(Widget) child(int index) inout { return _children.tryGet(index); }
     /// adds child, returns added item
     override Widget addChild(Widget item) { return _children.add(item).parent(this); }
     /// inserts child at given index, returns inserted item