register nfc callback in every activity that provides a proper sharable uri

src/main/java/eu/siacs/conversations/ui/ConferenceDetailsActivity.java

@@ -144,7 +144,7 @@ public class ConferenceDetailsActivity extends XmppActivity {
 		if (conversation!=null) {
 			return "xmpp:"+conversation.getContactJid().split("/")[0]+"?join";
 		} else {
-			return super.getShareableUri();
+			return "";
 		}
 	}
 

src/main/java/eu/siacs/conversations/ui/ContactDetailsActivity.java

@@ -166,7 +166,7 @@ public class ContactDetailsActivity extends XmppActivity {
 		if (contact!=null) {
 			return "xmpp:"+contact.getJid();
 		} else {
-			return super.getShareableUri();
+			return "";
 		}
 	}
 

src/main/java/eu/siacs/conversations/ui/ConversationActivity.java

@@ -116,7 +116,7 @@ public class ConversationActivity extends XmppActivity implements
 		if (conversation!=null) {
 			return "xmpp:"+conversation.getAccount().getJid();
 		} else {
-			return super.getShareableUri();
+			return "";
 		}
 	}
 
@@ -659,18 +659,6 @@ public class ConversationActivity extends XmppActivity implements
 		}
 	}
 
-	@Override
-	public void onResume() {
-		super.onResume();
-		this.registerNdefPushMessageCallback();
-	}
-
-	@Override
-	public void onPause() {
-		super.onPause();
-		this.unregisterNdefPushMessageCallback();
-	}
-
 	@Override
 	protected void onStop() {
 		if (xmppConnectionServiceBound) {

src/main/java/eu/siacs/conversations/ui/EditAccountActivity.java

@@ -287,7 +287,7 @@ public class EditAccountActivity extends XmppActivity {
 		if (mAccount!=null) {
 			return "xmpp:"+mAccount.getJid();
 		} else {
-			return super.getShareableUri();
+			return "";
 		}
 	}
 

src/main/java/eu/siacs/conversations/ui/XmppActivity.java

@@ -588,6 +588,20 @@ public abstract class XmppActivity extends Activity {
 		return null;
 	}
 
+	@Override
+	public void onResume() {
+		super.onResume();
+		if (this.getShareableUri()!=null) {
+			this.registerNdefPushMessageCallback();
+		}
+	}
+
+	@Override
+	public void onPause() {
+		super.onPause();
+		this.unregisterNdefPushMessageCallback();
+	}
+
 	protected void showQrCode() {
 		String uri = getShareableUri();
 		if (uri!=null) {