Leetcode 2493: Java code will have Null pointer exception

[AthulRajuGit]

java.lang.NullPointerException: Cannot invoke "java.util.List.iterator()" because the return value of "java.util.Map.get(Object)" is null
  at line 6, Solution.isBipartite
  at line 79, Solution.magnificentSets

which is caused by by

 adj.computeIfAbsent(u, k -> new ArrayList<>()).add(v);
 adj.computeIfAbsent(v, k -> new ArrayList<>()).add(u);

can be resolved by

adj.get(u).add(v);
adj.get(v).add(u);

This occurs because some nodes in the graph may not have any neighboring nodes, which can lead to null pointer exception in isBipartite() method