matchesJsonPath throws warning as 2020-04-24 15:49:47.220 Warning: JSON path expression '$.xxx'

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Bug reports

Added body in POST request as

"bodyPatterns": [
            { "matchesJsonPath": "$.xxx" },
            { "matchesJsonPath": "$.xxx" }
  ]

giving warning to all GET request as

2020-04-24 15:49:47.220 Warning: JSON path expression '$.xxx' failed to match document 'null' because of error 'json string can not be null or empty'

Please help we the issue