diff --git a/core/controllers/ApplicationVersionController.py b/core/controllers/ApplicationVersionController.py
index 7e4873b..1664722 100644
--- a/core/controllers/ApplicationVersionController.py
+++ b/core/controllers/ApplicationVersionController.py
@@ -29,7 +29,7 @@ class ApplicationVersionController:
         if not application_version.is_supported():
             raise UnsupportedApplicationVersionError('The application version in question is not supported.')
 
-        if reinstall is True:
+        if reinstall:
             ApplicationVersionController.uninstall(application_version)
 
         if application_version.is_installed():
diff --git a/core/controllers/ClientController.py b/core/controllers/ClientController.py
index 202f31a..97e216a 100644
--- a/core/controllers/ClientController.py
+++ b/core/controllers/ClientController.py
@@ -6,7 +6,7 @@ from core.controllers.ClientVersionController import ClientVersionController
 from core.controllers.ConfigurationController import ConfigurationController
 from core.controllers.LocationController import LocationController
 from core.controllers.SubscriptionPlanController import SubscriptionPlanController
-from core.observers import ClientObserver
+from core.observers.ClientObserver import ClientObserver
 from core.observers.ConnectionObserver import ConnectionObserver
 from typing import Optional
 import os
diff --git a/core/controllers/ConnectionController.py b/core/controllers/ConnectionController.py
index cb73391..92bda63 100644
--- a/core/controllers/ConnectionController.py
+++ b/core/controllers/ConnectionController.py
@@ -1,3 +1,4 @@
+from collections.abc import Callable
 from core.Constants import Constants
 from core.Errors import InvalidSubscriptionError, MissingSubscriptionError, ConnectionUnprotectedError, ConnectionTerminationError, CommandNotFoundError
 from core.controllers.ConfigurationController import ConfigurationController
@@ -10,7 +11,7 @@ from core.models.system.SystemState import SystemState
 from core.observers import ConnectionObserver
 from core.services.WebServiceApiService import WebServiceApiService
 from pathlib import Path
-from typing import Union, Optional
+from typing import Union, Optional, Any
 import os
 import re
 import requests
@@ -24,7 +25,7 @@ import time
 class ConnectionController:
 
     @staticmethod
-    def with_preferred_connection(*args, task: callable, connection_observer: Optional[ConnectionObserver] = None, **kwargs):
+    def with_preferred_connection(*args, task: Callable[..., Any], connection_observer: Optional[ConnectionObserver] = None, **kwargs):
 
         connection = ConfigurationController.get_connection()
 
@@ -34,6 +35,9 @@ class ConnectionController:
         elif connection == 'tor':
             return ConnectionController.__with_tor_connection(*args, task=task, connection_observer=connection_observer, **kwargs)
 
+        else:
+            return None
+
     @staticmethod
     def establish_connection(profile: Union[SessionProfile, SystemProfile], force: bool = False, connection_observer: Optional[ConnectionObserver] = None):
 
@@ -110,6 +114,8 @@ class ConnectionController:
                 else:
                     raise ConnectionError('The connection could not be established.')
 
+        return None
+
     @staticmethod
     def establish_session_connection(profile: SessionProfile, force: Optional[bool] = False, connection_observer: Optional[ConnectionObserver] = None):
 
@@ -360,7 +366,7 @@ class ConnectionController:
             network_interface_is_activated = ConnectionController.system_uses_wireguard_interface()
             attempt += 1
 
-        if network_interface_is_activated is False:
+        if not network_interface_is_activated:
             raise ConnectionError('The network interface could not be activated.')
 
     @staticmethod
@@ -375,7 +381,7 @@ class ConnectionController:
         return bool(re.search('dev wg', str(process_output)))
 
     @staticmethod
-    def __with_tor_connection(*args, task: callable, connection_observer: Optional[ConnectionObserver] = None, **kwargs):
+    def __with_tor_connection(*args, task: Callable[..., Any], connection_observer: Optional[ConnectionObserver] = None, **kwargs):
 
         session_directory = tempfile.mkdtemp(prefix='hv-')
         port_number = ConnectionController.get_random_available_port_number()
diff --git a/core/models/invoice/Invoice.py b/core/models/invoice/Invoice.py
index 111a729..d15cf21 100644
--- a/core/models/invoice/Invoice.py
+++ b/core/models/invoice/Invoice.py
@@ -1,4 +1,4 @@
-from core.models.invoice import PaymentMethod
+from core.models.invoice.PaymentMethod import PaymentMethod
 from dataclasses import dataclass
 from datetime import datetime
 
diff --git a/core/services/WebServiceApiService.py b/core/services/WebServiceApiService.py
index 92d0349..a0903c4 100644
--- a/core/services/WebServiceApiService.py
+++ b/core/services/WebServiceApiService.py
@@ -86,6 +86,9 @@ class WebServiceApiService:
         if response.status_code == requests.codes.created:
             return Subscription(response.headers['X-Billing-Code'])
 
+        else:
+            return None
+
     @staticmethod
     def get_subscription(billing_code: str, proxies: Optional[dict] = None):
 
@@ -100,6 +103,9 @@ class WebServiceApiService:
             subscription = response.json()['data']
             return Subscription(billing_code, Subscription.from_iso_format(subscription['expires_at']))
 
+        else:
+            return None
+
     @staticmethod
     def get_invoice(billing_code: str, proxies: Optional[dict] = None):
 
@@ -121,6 +127,9 @@ class WebServiceApiService:
 
             return Invoice(billing_code, invoice['status'], invoice['expires_at'], tuple[PaymentMethod](payment_methods))
 
+        else:
+            return None
+
     @staticmethod
     def get_proxy_configuration(billing_code: str, proxies: Optional[dict] = None):