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

src/dlangui/core/collections.d

@@ -42,7 +42,7 @@ module dlangui.core.collections;
 
 import std.algorithm;
 
-/** 
+/**
     Array based collection of items.
 
     Retains item order when during add/remove operations.
@@ -62,10 +62,10 @@ struct Collection(T, bool ownItems = false) {
     @property void size(size_t newSize) {
         if (_len > newSize)
             length = newSize; // shrink
-        _items.length = newSize; 
+        _items.length = newSize;
     }
     /// returns number of items in collection
-    @property void length(size_t newSize) { 
+    @property void length(size_t newSize) {
         if (newSize < _len) {
             // shrink
             static if (is(T == class) || is(T == struct)) {
@@ -248,6 +248,13 @@ 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 {
+        if (index < 0 || index >= _count) {
+            return null;
+        }
+        return _list[index];
+    }
     /// 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