How to fill child table of existing doctype

Frappe Framework Customization
1

I have requirement which is when i click a link i want to open a instance of doc-type which i created before
and fill it child table with given data. to make it clear i will produce an example

i have added 4 batches in student batch doc type

https://discuss.frappe.io/uploads/default/original/2X/f/f555b8ed6329e13c512df7f567b090a336682525.png

and i have other doc-type student applicant with list of students

Pasted image946×471 31.5 KB

when i select list of students i want to add selected students to student batch App Builder-TEST BATCH-2015-16

and fill the child table with selected student details.

i have already done adding student details to new instance of student batch
with help of @rohit_w

but my new requirement is add to existing student batch

i have tried some thing but i found no help with that i will share what i tried below

frappe.model.with_doctype("Student Batch", function() {
					var tlb = frappe.model.get_doc("Student Batch Student");
					console.log(tlb);
					$.each(stud_id, function(i, d) {
						var detail = frappe.model.get_doc("Student Batch Student", tlb,
							"students");

						$.extend(detail, {
							"student": d,
							"student_name": stud_name[i],
						});
					})
					frappe.set_route("Form", "Student Batch/App Builder-TEST BATCH-2015-16",);
				})

Above code opens student batch App Builder-TEST BATCH-2015-16 but no thing is added to child table

but when changed above code to

 frappe.model.with_doctype("Student Batch", function() {
    					var tlb = frappe.model.get_doc("Student Batch Student");
    					console.log(tlb);
    					$.each(stud_id, function(i, d) {
    						var detail = frappe.model.get_doc("Student Batch Student", tlb,
    							"students");

    						$.extend(detail, {
    							"student": d,
    							"student_name": stud_name[i],
    						});
    					})
    					frappe.set_route("Form", "Student Batch/App Builder-TEST BATCH-2015-16",tlb.name);
    				})

i am getting an error in console

VM311:127 Uncaught TypeError: Cannot read property ‘name’ of null(…)

can an one please help me