fixed bug in get_child_indices function

tree.go

@@ -37,6 +37,10 @@ func get_child_indices(header_index int, headings []heading) []int {
 
 	parent_level := get_heading_level(headings[header_index].text)
 
+	if header_index >= (len(headings) - 1) {
+		return nil
+	}
+
 	next_level := get_heading_level(headings[header_index+1].text)
 
 	child_level := 0