Pulling Top queries returns NULL in query plan and sql text
There are limitations when using sys.dm_exec_query_plan
5.4 When and How to Use Sys.dm_exec_text_query_plan

The query_plan column returned by sys.dm_exec_query_plan has type XML and suffers from the inherent limitation of the data type that it cannot have nest levels greater than or equal to 128. In SQL Server 2005 RTM and SP1, if the query_plan had a depth of greater than or equal to 128 levels this would prevent the query from returning. The second limitation is that it is not easy to get the query plan of a particular statement in the batch directly from the DMVs without parsing the query plan XML returned from sys.dm_exec_query_plan.
To address both these limitations, in SQL Server 2005 SP2, we introduced sys.dm_exec_text_query_plan. Sys.dm_exec_text_query_plan is a TVF that takes 3 parameters: plan_handle, statement_start_offset and statement_end_offset. It returns the showplan in text format for the batch or for a specific statement within the batch.

Instead of reinventing the wheel, best is to use Brent's - sp_BlitzCache or Glenn's Diagnostic queries. or Aaron's answer here.
From SQL Server Query Performance Analysis using DMVs :
-- Which Queries are taking the most time/cpu to execute
SELECT TOP 20
    total_worker_time, total_elapsed_time,
    total_worker_time/execution_count AS avg_cpu_cost, execution_count,
    (SELECT DB_NAME(dbid) + ISNULL('..' + OBJECT_NAME(objectid), '')
        FROM sys.dm_exec_sql_text([sql_handle])) AS query_database,
    (SELECT SUBSTRING(est.[text], statement_start_offset/2 + 1,
        (CASE WHEN statement_end_offset = -1
            THEN LEN(CONVERT(nvarchar(max), est.[text])) * 2
            ELSE statement_end_offset
            END - statement_start_offset) / 2
        )
        FROM sys.dm_exec_sql_text([sql_handle]) AS est) AS query_text,
    total_logical_reads/execution_count AS avg_logical_reads,
    total_logical_writes/execution_count AS avg_logical_writes,
    last_worker_time, min_worker_time, max_worker_time,
    last_elapsed_time, min_elapsed_time, max_elapsed_time,
    plan_generation_num, qp.query_plan
FROM sys.dm_exec_query_stats
    OUTER APPLY sys.dm_exec_query_plan([plan_handle]) AS qp
WHERE [dbid] &gt;= 5 AND DB_NAME(dbid) IS NOT NULL
  AND (total_worker_time/execution_count) &gt; 100
--ORDER BY avg_cpu_cost DESC;
--ORDER BY execution_count DESC;
ORDER BY total_worker_time DESC;