When the value of "operator" is "=" "value" must not be null or empty.

It report error with this which have been hinder me for couple days.

Interface Definition: Expression evaluation error at function a!queryLogicalExpression [line 25]: The a!queryFilter function has an invalid value for the "value" parameter. When the value of "operator" is "=" "value" must not be null or empty.

a!localVariables(
a!formLayout(
  label: "Username",
  contents: {
    a!textField(
      label: "username",
      labelPosition: if(
        ri!readOnly,
        "ADJACENT",
        "ABOVE"
      ),
      value: ri!username.username,
      saveInto: ri!username.username,
      refreshAfter: "UNFOCUS",
      validations: {
        if(
          a!queryEntity(
            entity: cons!usernameConstant,
            query: a!query(
              fetchTotalCount: true,
              pagingInfo: a!pagingInfo(
                startIndex: 1,
                batchSize: 1
              ),
              logicalexpression: a!queryLogicalExpression(
                operator:"OR",
              filters: {
              a!queryFilter(
                field: "username",
                operator: "=",
                value:(ri!username.username)
              ),
              a!queryFilter(
                field: "username",
                operator: "is null"
              ),
              }
              )
            )
          ).totalCount > 0,
          "Username exists",
          null
        )

      }
    ),
    a!textField(
      label: "gender",
      labelPosition: if(
        ri!readOnly,
        "ADJACENT",
        "ABOVE"
      ),
      value: ri!username.gender,
      saveInto: ri!username.gender,
      refreshAfter: "UNFOCUS",
    ),
    a!columnsLayout(
      columns: {
        a!columnLayout(
          contents: {
            a!textField(
              label: "Password",
              labelPosition: if(
                ri!readOnly,
                "ADJACENT",
                "ABOVE"
              ),
              value: ri!username.password,
              saveInto: ri!username.password,
              characterLimit: 255,
              readOnly: ri!readOnly
            )
          }
        )
      }
    ),
  },
 
  buttons: a!buttonLayout(
    primaryButtons: {
      a!buttonWidget(
        label: "Insert",
        
        saveInto:(
          a!writeToDataStoreEntity(
            dataStoreEntity: cons!usernameConstant,
            valueToStore:(ri!username)
           ),       
           ),
           
        submit: true,
        style: "PRIMARY",
      ),
      a!buttonWidget(
        label: "update",
        saveInto:(
          a!writeToDataStoreEntity(
            dataStoreEntity: cons!usernameConstant,
            valueToStore:ri!username,           
          ),
        ),
        submit: true,
        style: "PRIMARY"
      ),
      a!buttonWidget(
        label: "delete",      
        saveInto:(
          a!deleteFromDataStoreEntities(
            dataToDelete: {
              a!entityDataIdentifiers(
                entity: cons!usernameConstant,
                identifiers:{ri!username.username},
              )
            }
          )
        ),
        submit: true,
        style: "PRIMARY"
      )
    },
    secondaryButtons: {
      a!buttonWidget(
        label: "Cancel",
        value: true,
        saveInto: ri!cancel,
        submit: true,
        style: "NORMAL",
        validate: false
      )
    },
    showWhen: or(
      isnull(
        ri!readOnly
      ),
      not(
        ri!readOnly
      )
    )
  )
 
)
)