fix pubkey regex for mobile medium groups

ts/receiver/contentMessage.ts

@@ -80,7 +80,7 @@ async function decryptForMediumGroup(
     window.console.info(
       'Dropping message from ourself after decryptForMediumGroup'
     );
-    return;
+    return null;
   }
 
   const plaintext = await decryptWithSenderKey(

ts/session/messages/outgoing/content/data/group/ClosedGroupMessage.ts

@@ -32,7 +32,12 @@ export abstract class ClosedGroupMessage extends DataMessage {
   }
 
   protected groupContext(): SignalService.GroupContext {
-    const id = new Uint8Array(StringUtils.encode(this.groupId.key, 'utf8'));
+    let groupIdWithPrefix: string = this.groupId.key;
+    if (!this.groupId.key.startsWith(PubKey.PREFIX_GROUP_TEXTSECURE)) {
+      groupIdWithPrefix = PubKey.PREFIX_GROUP_TEXTSECURE + this.groupId.key;
+    }
+    const encoded = StringUtils.encode(groupIdWithPrefix, 'utf8');
+    const id = new Uint8Array(encoded);
 
     return new SignalService.GroupContext({ id });
   }

ts/session/types/PubKey.ts

@@ -8,7 +8,6 @@ export class PubKey {
   // tslint:disable: member-ordering
   public static readonly regexForPubkeys = `((05)?${PubKey.HEX}{64})`;
   public static readonly PREFIX_GROUP_TEXTSECURE = '__textsecure_group__!';
-
   // prettier-ignore
   private static readonly regex: RegExp = new RegExp(
     `^(${PubKey.PREFIX_GROUP_TEXTSECURE})?(05)?(${PubKey.HEX}{64}|${PubKey.HEX}{32})$`