restrictions apparantly cannot have a null default value

src/Android/Resources/xml/app_restrictions.xml

@@ -5,5 +5,5 @@
     android:title="@string/SelfHostedServerUrl"
     android:restrictionType="string"
     android:description="@string/ServerUrl"
-    android:defaultValue="@null" />
+    android:defaultValue="" />
 </restrictions>

src/App/Utilities/AppHelpers.cs

@@ -163,7 +163,7 @@ namespace Bit.App.Utilities
                         {
                             await environmentService.SetUrlsAsync(new Core.Models.Data.EnvironmentUrlData
                             {
-                                Base = setting.Value,
+                                Base = string.IsNullOrWhiteSpace(setting.Value) ? null : setting.Value,
                                 Api = environmentService.ApiUrl,
                                 Identity = environmentService.IdentityUrl,
                                 WebVault = environmentService.WebVaultUrl,