Android test in kotlin of realm

How can be done a simple test of a realm database in Android implementing the test in Kotlin?

I attempted to adapt a fragment from java realm test on github to kotlin and got the next code:

import io.realm.Realm 
import io.realm.log.RealmLog 
import org.hamcrest.CoreMatchers 
import org.junit.Assert

import org.junit.Test import org.junit.Before import org.junit.Rule
import org.mockito.Mockito.`when` 
import org.powermock.api.mockito.PowerMockito 
import org.powermock.modules.junit4.rule.PowerMockRule

class DBTest {

    @Rule
    var rule = PowerMockRule()
    lateinit internal var mockRealm: Realm

    @Before
    fun setup() {
        PowerMockito.mockStatic(RealmLog::class.java)
        PowerMockito.mockStatic(Realm::class.java)

        val mockRealm = PowerMockito.mock(Realm::class.java)

        `when`(Realm.getDefaultInstance()).thenReturn(mockRealm)

        this.mockRealm = mockRealm
    }

    @Test
    fun shouldBeAbleToGetDefaultInstance() {
        Assert.assertThat(Realm.getDefaultInstance(), CoreMatchers.`is`(mockRealm))
    }

}

But when I execute the test I get:

org.junit.internal.runners.rules.ValidationError: The @Rule 'rule' must be public.