Fix real Usb Device references in Printer

src/printers/P700Printer.cpp

@@ -138,6 +138,10 @@ bool P700Printer::printBitmap(const graphics::Bitmap<graphics::ALPHA8>& bitmap)
 }
 
 bool P700Printer::printMonochromeData(const graphics::MonochromeData& data) {
+    if (!mUsbHndl) {
+        spdlog::error("USB Handle is invalid!");
+        return false;
+    }
     // Send initialization sequence
     // The INITIALIZE command needs to be sent as a 128-byte packet with ESC @ at the end
     std::vector<uint8_t> initCmd(128, 0x00);