"unexpectedly found nil while unwrapping an Optional value" in DidDeselectRowAtIndexpath

I know this is an error a lot of people get and I know what it means, but for this matter i'm not able to fix/debug it. I have a table view with a custom cell. Once you select a row, some things will happen. Once the row gets deselected, some other things will happen.

This all works fine, but there is one bug I can't fix. I can tap on every row I want and 'nothing' happens, but if I tap on the second row and then the last row or the other way around I get this error:

unexpectedly found nil while unwrapping an Optional value

The LLDB showed me where the error occurred and it should be in this line of code:

let cell = tableView.cellForRowAtIndexPath(indexPath) as! TaskViewerCell

Is there anybody who knows how to fix this issue? I've added the code of both functions below.

Thanks

override func tableView(tableView: UITableView, didSelectRowAtIndexPath indexPath: NSIndexPath) {
    let cell = tableView.cellForRowAtIndexPath(indexPath) as! TaskViewerCell

    number = indexPath.row
    if cell.submit.hidden != false || next[0..<indexPath.row].contains(false){
        cell.submit.setTitle("Not completed", forState: .Normal)
        cell.submit.setTitleColor(UIColor.orangeColor(), forState: .Normal)
        logbook = "Pending"
    }else{
        logbook = "completed"
        cell.submit.setTitle("Completed", forState: .Normal)
        cell.submit.setTitleColor(UIColor.greenColor(), forState: .Normal)
    }
    if indexPath.row != 0 && next[indexPath.row-1] == true{
    cell.nSwitch.enabled = true
    cell.nSegment.enabled=true
    cell.nButton.enabled = true
    cell.submit.enabled = true
        print("tapped(indexPath.row)")
    }else{
        cell.nSwitch.enabled = false
        cell.nSegment.enabled=false
        cell.nButton.enabled = false
        cell.submit.enabled = false
    }

}

override func tableView(tableView: UITableView, didDeselectRowAtIndexPath indexPath: NSIndexPath) {
    let cell = tableView.cellForRowAtIndexPath(indexPath) as! TaskViewerCell // This is where is should go wrong!!
    switch1[indexPath.row] = cell.nSwitch.on
    yesno1[indexPath.row] = cell.nSegment.selectedSegmentIndex
    if cell.nField != "" {
            field1[indexPath.row] = cell.nField.text!
          }else{
            field1[indexPath.row] = cell.nField.placeholder!
        }
    topField[indexPath.row] = cell.topField.text!
    topSegment[indexPath.row] = cell.topSegment.selectedSegmentIndex

    cell.nSwitch.enabled = false
    cell.nSegment.enabled = false
    cell.nButton.enabled = false
    cell.submit.enabled = false

}