Fix for both Uthernet cards not reading I/O $Csxx space as floating-bus. (#1321)

source/Uthernet1.cpp

@@ -37,6 +37,7 @@
 #include "Tfe/tfesupp.h"
 #include "Tfe/NetworkBackend.h"
 #include "Tfe/PCapBackend.h"
+#include "../Windows/Win32Frame.h"
 
 /* Makros for reading and writing the visible TFE register: */
 #define GET_TFE_8(  _xxx_ ) \
@@ -1017,6 +1018,13 @@ void Uthernet1::InitializeIO(LPBYTE pCxRomPeripheral)
     {
         RegisterIoHandler(m_slot, TfeIo, TfeIo, TfeIoCxxx, TfeIoCxxx, this, NULL);
     }
+    else
+    {
+        // Interface doesn't exist or user picked an interface that isn't Ethernet!
+        // . So setup as a "null" card: I/O reads from floating bus & writes go to null
+        RegisterIoHandler(m_slot, IO_Null, IO_Null, TfeIoCxxx, TfeIoCxxx, this, NULL);
+        Win32Frame::GetWin32Frame().FrameMessageBox("Reconfigure the Interface via 'Ethernet Settings'.", "Uthernet 1 interface isn't valid!", MB_ICONEXCLAMATION | MB_SETFOREGROUND);
+    }
 }
 
 void Uthernet1::Reset(const bool powerCycle)