供参考建造者似乎与建造者模式有关，你可能想阅读它，看看你是否能理解你的项目中的某些东西。

If both classes are at same level (Both are child class), how to use instance of one class into another one.What is best way to use instance of one class into another class without passing into constructor? so manually require to pass null. How to make independent code?

    Class PreviewPanel{

    private PreviewPanel(Builder builder) {
        this.previewMode=builder.previewMode;

        formsPreview=new NTFormsPreview(previewMode);
        formsPreview.setCanAddComment(builder.canAddComment);

        ntPreviewTreePanel=new NTPreviewTreePanel(builder,formsPreview);
    //This class have some event bus implemented.Sometime There, formsPreview instance is require.

           }
    public static class Builder {
        private PreviewMode previewMode;

        private Document document;
        public Builder(PreviewMode previewMode,Document document) {
            this.previewMode = previewMode;
            this.document=document;
        }
        public PreviewPanel build() {
              return new PreviewPanel(this);
        }

     }
}

If I pass that instance into constructor,I have to follow chain of inner class and pass same instance to reach specific class. I want to avoid it. This is big product. it is not easy to show how many classes inside it to reach actual handler implementation.

Code Structure:

private PreviewPanel(Builder builder)
    ->formsPreview=new NTFormsPreview(previewMode);

        ->NTPreviewTreePanel(builder,formsPreview);
                 ->NTPreviewTree(document, bidDocuments, previewMode, canAddComment,canViewComment, previewFormTxnEncryptionDetails,formsPreview);
             ->NTTreeNode(formsPreview)
                private void fireReportItemClicked(Document document,esenderCSReport){
                                eventBus.fireEventFromSource(formPreviewEvent, formsPreview);   
                }

Your problem is unclear to me but I think you could do the following :

Create a new Class that will "Own both of your child class - composition"

Make it work a Mediator Pattern so that one can use and call stuff on the other one following the rule/logic you want.

1. No need to pass one of the child to the other one in any way.
2. Freely, redesign the interaction logic if it gets to change
3. No need to "change"" the structure you already have

FYI The builder, seems to be related to the Builder Pattern, you might wanna read on it and see if you can understand something out of your project.