FIX: fix a crash, set APP_MODAL for dialog

Change-Id: I80d216df134febdcefd2fd5f3859bdb62c1b81bd
Signed-off-by: Stone Li <stone.li@bambulab.com>

src/slic3r/GUI/DeviceManager.cpp

@@ -300,7 +300,7 @@ void MachineObject::set_access_code(std::string code)
 bool MachineObject::is_lan_mode_printer()
 {
     bool result = false;
-    if (connection_type() == "lan")
+    if (!dev_connection_type.empty() && dev_connection_type == "lan")
         return true;
     return result;
 }