In earlier versions of the cartridge, the database agent displayed "SQL Handle: null" because the blck_sql_handle field in pass_request_stat_fact in the repository is empty. For display reasons, the coding changed it to "Unknown batch" so the (sqlkey=null) link would not work.
The SQL text can be been flushed away by SQL Server between agent collections. This is just a side effect of defined time gap between Top SQL collections and SQL Server clearing away cache and sometimes the agent misses getting results from SQL Server when querying sys.dm_exec_sql_text.