To reproduce, execute this line in a notebook (runtime 15.3):
dbutils.widgets.multiselect("foo", None, [None])
Exception raised:
Py4JJavaError: An error occurred while calling o427.createMultiselectWidget. : java.lang.NullPointerException at com.databricks.backend.daemon.driver.NotebookArguments.trimDefaultValue(NotebookArguments.scala:88)