diff --git a/src/org/kde/kdeconnect/Helpers/ContactsHelper.java b/src/org/kde/kdeconnect/Helpers/ContactsHelper.java
index 5f2a40e2..899ad152 100644
--- a/src/org/kde/kdeconnect/Helpers/ContactsHelper.java
+++ b/src/org/kde/kdeconnect/Helpers/ContactsHelper.java
@@ -46,6 +46,7 @@ import java.util.List;
 import java.util.Map;
 import java.util.Set;
 
+import androidx.annotation.NonNull;
 import androidx.annotation.RequiresApi;
 import androidx.collection.LongSparseArray;
 
@@ -394,6 +395,7 @@ public class ContactsHelper {
                     .append("\n");
         }
 
+        @NonNull
         public String toString() {
             return vcardBody.toString() + VCARD_END;
         }
@@ -415,9 +417,14 @@ public class ContactsHelper {
         static final String COLUMN = ContactsContract.Contacts.LOOKUP_KEY;
 
         public uID(String lookupKey) {
+
+            if (lookupKey == null)
+                throw new IllegalArgumentException("lookUpKey should not be null");
+
             contactLookupKey = lookupKey;
         }
 
+        @NonNull
         public String toString() {
             return this.contactLookupKey;
         }
diff --git a/src/org/kde/kdeconnect/Plugins/SftpPlugin/RootFile.java b/src/org/kde/kdeconnect/Plugins/SftpPlugin/RootFile.java
index fa94245a..a6eea5dc 100644
--- a/src/org/kde/kdeconnect/Plugins/SftpPlugin/RootFile.java
+++ b/src/org/kde/kdeconnect/Plugins/SftpPlugin/RootFile.java
@@ -22,6 +22,7 @@ package org.kde.kdeconnect.Plugins.SftpPlugin;
 
 import org.apache.sshd.common.file.SshFile;
 
+import java.io.IOException;
 import java.io.InputStream;
 import java.io.OutputStream;
 import java.util.Calendar;