How to insert List in room database?

Question

I am using LocalDateTime from kotlinx-datetime library. Want to insert List<LocalDateTime> in Room database. Typeconverter written using Gson library. But following error showing.

java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String java.time.LocalDate.toString()' on a null object reference
                                                                                                        at java.time.LocalDateTime.toString(LocalDateTime.java:1966)
                                                                                                        at java.lang.String.valueOf(String.java:2924)
                                                                                                        at java.lang.StringBuilder.append(StringBuilder.java:132)
                                                                                                        at com.quixcure.pillu.data.model.DoseTiming.toString(Unknown Source:27)
                                                                                                        at java.lang.String.valueOf(String.java:2924)
                                                                                                        at java.lang.StringBuilder.append(StringBuilder.java:132)
                                                                                                        at java.util.AbstractCollection.toString(AbstractCollection.java:462)
                                                                                                        at java.lang.String.valueOf(String.java:2924)
                                                                                                        at org.junit.Assert.format(Assert.java:844)
                                                                                                        at org.junit.Assert.failNotEquals(Assert.java:835)
                                                                                                        at org.junit.Assert.assertEquals(Assert.java:120)
                                                                                                        at org.junit.Assert.assertEquals(Assert.java:146)