[AX25] Added check for Bell modem

src/protocols/AX25/AX25.cpp

@@ -194,7 +194,12 @@ int16_t AX25Client::begin(const char* srcCallsign, uint8_t srcSSID, uint8_t preL
   preambleLen = preLen;
 
   // configure for direct mode
-  return(phyLayer->startDirect());
+  #if !defined(RADIOLIB_EXCLUDE_AFSK)
+  if(bellModem != nullptr) {
+    return(phyLayer->startDirect());
+  }
+  #endif
+  return(RADIOLIB_ERR_NONE);
 }
 
 #if defined(RADIOLIB_BUILD_ARDUINO)