Fix more compilation errors.

internal/platform/implementation/linux/bluetooth_devices.cc

@@ -3,10 +3,10 @@
 
 #include <sdbus-c++/Types.h>
 
+#include "absl/synchronization/mutex.h"
 #include "internal/platform/implementation/linux/bluetooth_classic_device.h"
 #include "internal/platform/implementation/linux/bluetooth_devices.h"
 #include "internal/platform/implementation/linux/bluez.h"
-#include "absl/synchronization/mutex.h"
 
 namespace nearby {
 namespace linux {
@@ -19,8 +19,8 @@ BluetoothDevices::get_device_by_path(
     return std::nullopt;
   }
 
-  auto &device = devices_by_path_[device_object_path];
-  return device;
+  auto &device = devices_by_path_.at(device_object_path);
+  return *device;
 }
 
 std::optional<std::reference_wrapper<BluetoothDevice>>
@@ -40,10 +40,11 @@ void BluetoothDevices::remove_device_by_path(
 BluetoothDevice &
 BluetoothDevices::add_new_device(sdbus::ObjectPath device_object_path) {
   absl::MutexLock l(&devices_by_path_lock_);
-  auto pair =
-      devices_by_path_.emplace(device_object_path, system_bus_,
-                               std::move(device_object_path), observers_);
-  return pair.first->second;
+  auto pair = devices_by_path_.emplace(
+      std::string(device_object_path),
+      std::make_unique<MonitoredBluetoothDevice>(
+          system_bus_, std::move(device_object_path), observers_));
+  return *pair.first->second;
 }
 } // namespace linux
 } // namespace nearby