[Baertram]

I'm not using MM and won't test nor support the addon in any way, but it may work and help:

Try to change the code in the file MasterMerchant.lua at lines 2324ff to the following:

function MasterMerchant:initBuyingAdvice()
  if MasterMerchant.originalSetupCallback then return end
--Inserted by Baertram - START
  if TRADING_HOUSE.m_searchResultsList == nil then return false end
--Inserted by Baertram - END
  local dataType = TRADING_HOUSE.m_searchResultsList.dataTypes[1]
 
  MasterMerchant.originalSetupCallback = dataType.setupCallback
  if MasterMerchant.originalSetupCallback then
      dataType.setupCallback = function(...)
          local row, data = ...
          MasterMerchant.originalSetupCallback(...)
          zo_callLater(function() MasterMerchant.AddBuyingAdvice(row, data) end, 25)
      end
  else
      d(GetString(MM_ADVICE_ERROR))
  end
end

And in lines 2272ff then:

function MasterMerchant:initSellingAdvice()
  if MasterMerchant.originalSellingSetupCallback then return end
--Inserted by Baertram - START
  if TRADING_HOUSE.m_postedItemsList == nil then return false end
--Inserted by Baertram - END
  local dataType = TRADING_HOUSE.m_postedItemsList.dataTypes[2]
 
  MasterMerchant.originalSellingSetupCallback = dataType.setupCallback
  if MasterMerchant.originalSellingSetupCallback then
      dataType.setupCallback = function(...)
          local row, data = ...
          MasterMerchant.originalSellingSetupCallback(...)
          zo_callLater(function() MasterMerchant.AddSellingAdvice(row, data) end, 25)
      end
  else
      d(GetString(MM_ADVICE_ERROR))
  end
end

I'm not sure what the addon is adding there (Buying/Selling advice) but the funcitons might not work with my changes then.