If table is empty, insert null value, else carry on.

I figured it out.... the table name is the name of the previous step. I was calling it by a different name. It's always a small problem isn't it.... anyway thanks for your help!

The end of the code now reads:

#"Removed Other Columns3" = Table.SelectColumns(#"Renamed Columns2",{"BatchNo"}),
    #"Changed Type3" = Table.TransformColumnTypes(#"Removed Other Columns3",{{"BatchNo", type text}}),
    #"AddNull" = if Table.IsEmpty(#"Changed Type3") then Table.InsertRows(#"Changed Type3",0, { [BatchNo = "null"] }) else #"Changed Type3",
    #"Search Batch Number" = #"AddNull"{0}[BatchNo]
in
    #"Search Batch Number"

All I had to do was change Table.InsertRows(BatchNumber2, 0, { [BatchNo = "null"] }) to Table.InsertRows(#"Changed Type3", 0, { [BatchNo = "null"] }).