Google Script import txt files from Google Drive - arrays

Question

so I'm trying to write a script to import text files, based on the file names, and the tab names.
The txt files are game server log extracts, 3 different types of logs.
I have managed to get the script to import the text based on the file names (types), but having issues running some simple formatting at the end of a loop before moving to the next type or log.
Basically, the script imports all the text from the log files parses it as a CSV and dumps it to the sheet. The sheet removes duplicates, splits the text into columns etc, they should move on to the next type of log (and switch tab) - repeat.
I'm still picking scripting up so please be gentle....! Any help is greatly appreciated.
function ImportAll() {
var folderID = "my google drive foler"
var folder = DriveApp.getFolderById(folderID);
var ss, sRow, lRow, fRow, csvData, logFile, file, sheet =
SpreadsheetApp.getActive();
var data = [];
var logType = ["admin_", "kill_", "login_"]
var allSheets = ['Admin', 'Kill', 'Login']
for (var k = 0; k < logType.length; k++) {
var files = folder.searchFiles('title contains "' + logType[k] + '"');
while (files.hasNext()) {
file = files.next();
data.push(file.getName())
};
ss = sheet.getSheetByName(allsheets[k])
sRow = ss.getDataRange().getLastRow() + 1;
//Browser.msgBox(data[0])
for (var i = 0; i < data.length; i++) {
logFile = DriveApp.getFilesByName(data[i]).next();
csvData = Utilities.parseCsv(logFile.getBlob().getDataAsString());
fRow = ss.getDataRange().getLastRow() + 1;
ss.getRange(fRow, 1, csvData.length, csvData[0].length).setValues(csvData);
};
lRow = ss.getDataRange().getLastRow();
ss.getRange('A' + sRow + ':A' + lRow).removeDuplicates().activate();
ss.getRange('A' + sRow + ':A' + lRow).splitTextToColumns('-');
ss.getRange('B' + sRow + ':B' + lRow).splitTextToColumns(':');
ss.getRange('B:B').setNumberFormat('HH:mm:ss');
ss.getFilter().remove();
ss.getRange('A1:E' + lRow).createFilter();
var rng = sheet.getRange('D2:D' + lRow)
var rngV = rng.getValues();
var String = "";
for(var i=0;i<rngV.length;i++)
{
String = rngV[i].toString().replace(s, '')
rngV[i] = String // is working
}
rng.setValues(rngV) // NOT WORKING!!!!!!
//sheet.appendRow(data); //throws [L]JavaLang#****
}
};
}
I keep getting thrown "TypeError: Cannot call method "getDataRange" of null." errors, and I've tried a heap of different things to no avail.