fix: Bump realm-gradle-plugin to 6.1.0 and fix build

Author: rob729

build.gradle

@@ -9,7 +9,7 @@ buildscript {
         classpath "com.android.tools.build:gradle:3.5.3"
         classpath "com.jfrog.bintray.gradle:gradle-bintray-plugin:1.8.4"
         classpath "com.github.dcendents:android-maven-gradle-plugin:2.1"
-        classpath "io.realm:realm-gradle-plugin:5.14.0"
+        classpath "io.realm:realm-gradle-plugin:6.1.0"
         classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:1.3.61"
 
         // NOTE: Do not place your application dependencies here; they belong
@@ -19,6 +19,7 @@ buildscript {
 
 allprojects {
     repositories {
+        jcenter()
         google()
     }