[Classes] Non nullable id and mxcurl

lib/src/User.dart

@@ -66,13 +66,13 @@ class User {
   MxContent get avatar_url => avatarUrl;
 
   User(
-    this.id, {
+    String id, {
     this.membership,
     this.displayName,
     this.avatarUrl,
     this.powerLevel,
     this.room,
-  });
+  }) : this.id = id ?? "";
 
   /// Returns the displayname or the local part of the Matrix ID if the user
   /// has no displayname.

lib/src/utils/MxContent.dart

@@ -29,7 +29,7 @@ class MxContent {
   final String _mxc;
 
   /// Insert a mxc:// uri here.
-  MxContent(this._mxc);
+  MxContent(String mxcUrl) : this._mxc = mxcUrl ?? "";
 
   /// Returns the mxc uri.
   get mxc => _mxc;