AppleWatch app crashes with "this class is not key value coding-compliant" upon leaving the view

I have a simple table with three kinds of cells in my AppleWatch app. A waiting one, an error one and a multi cell result one. The table is correctly shown but when I click a cell to access the detail page I get this funny crash without the control ever entering my code again. If I remove the call to function:

    func setActivity(){
        let firstRow = myTable?.rowController(at:0)
        print("first row class=\(firstRow.self)")
        if firstRow == nil{
            self.myTable?.setNumberOfRows(1, withRowType: "WKWaitingRow")
        }
    }

that would have loaded the WKWaitingRow cell, the crash does not occur any longer - including the firt lin -, notwithstanding of course the other two kinds of cells remain.

This is the crash log:

Exception caught during invocation of received message, dropping incoming message and invalidating the connection.
Exception: [<inArrivoHDWatch_Extension.WKWaitingRow 0x15545f80> valueForUndefinedKey:]: this class is not key value coding-compliant for the key myTable.
[<inArrivoHDWatch_Extension.WKWaitingRow 0x15545f80> valueForUndefinedKey:]: this class is not key value coding-compliant for the key myTable.
(
  0   CoreFoundation                      0x1f39837b <redacted> + 152
  1   libobjc.A.dylib                     0x1ea471c3 objc_exception_throw + 38
  2   CoreFoundation                      0x1f398065 <redacted> + 0
  3   Foundation                          0x1fc8fe69 <redacted> + 260
  4   Foundation                          0x1fbeaa83 <redacted> + 226
  5   WatchKit                            0x2cd602f1 <redacted> + 4234
  6   WatchKit                            0x2cd5f251 <redacted> + 168
  7   CoreFoundation                      0x1f39d0e4 <redacted> + 84
  8   CoreFoundation                      0x1f2c5275 <redacted> + 296
  9   Foundation                          0x1fdbb611 <redacted> + 22
  10  Foundation                          0x1fc51e5b <redacted> + 1042
  11  Foundation                          0x1fbe6923 <redacted> + 750
  12  libxpc.dylib                        0x1f0d2221 <redacted> + 40
  13  libxpc.dylib                        0x1f0d068d <redacted> + 1164
  14  libdispatch.dylib                   0x1ee9c023 <redacted> + 8
  15  libdispatch.dylib                   0x1eeb2a01 <redacted> + 264
  16  libdispatch.dylib                   0x1eea8613 <redacted> + 426
  17  libdispatch.dylib                   0x1eeb33ef <redacted> + 566
  18  libdispatch.dylib                   0x1eea8613 <redacted> + 426
  19  libdispatch.dylib                   0x1eea8d21 <redacted> + 546
  20  libdispatch.dylib                   0x1eea98e7 <redacted> + 194
  21  libdispatch.dylib                   0x1eeb137b <redacted> + 768
  22  libsystem_pthread.dylib             0x1f09d4d3 _pthread_wqthread + 778
  23  libsystem_pthread.dylib             0x1f09d1a8 start_wqthread + 12
)