Remove null from getChildByName for missing child

This matches the behavior of Flash - no error is produced.

Author: Aaron1011

core/src/avm2/globals/flash/display/displayobjectcontainer.rs

@@ -168,14 +168,12 @@ pub fn get_child_by_name<'gc>(
             .cloned()
             .unwrap_or(Value::Undefined)
             .coerce_to_string(activation)?;
-        let child = dobj.child_by_name(&name, false).ok_or_else(|| {
-            format!(
-                "RangeError: Display object container has no child with name {}",
-                name
-            )
-        })?;
-
-        return Ok(child.object2());
+        if let Some(child) = dobj.child_by_name(&name, false) {
+            return Ok(child.object2());
+        } else {
+            log::warn!("Display object container has no child with name {}", name);
+            return Ok(Value::Null);
+        }
     }
 
     Ok(Value::Undefined)

tests/tests/swfs/avm2/displayobjectcontainer_getchildbyname/output.txt

@@ -3,5 +3,7 @@
 //this.getChildByName("child_clip").child_method()
 //Child method called
 //this.addChild(newChild)
-//this.getchildByName("new_child")
+//this.getChildByName("new_child")
 [object Sprite]
+//this.getChildByName("nonexistent_child")
+null