With mockito

ExampleTest.kt

import com.dropbox.android.external.store4.Fetcher
import com.dropbox.android.external.store4.Store
import com.dropbox.android.external.store4.StoreBuilder
import com.dropbox.android.external.store4.fresh
import com.dropbox.android.sample.Graph
import com.dropbox.android.sample.data.model.Post
import com.nhaarman.mockitokotlin2.any
import com.nhaarman.mockitokotlin2.mock
import com.nhaarman.mockitokotlin2.whenever
import kotlinx.coroutines.ExperimentalCoroutinesApi
import kotlinx.coroutines.FlowPreview
import kotlinx.coroutines.test.TestCoroutineScope
import kotlinx.coroutines.test.runBlockingTest
import org.junit.Before
import org.junit.Test
import org.junit.runner.RunWith
import org.junit.runners.JUnit4
import kotlin.time.ExperimentalTime

@OptIn(
        FlowPreview::class,
        ExperimentalCoroutinesApi::class,
        ExperimentalTime::class,
        ExperimentalStdlibApi::class
)
open class StoreProvider() {

    open fun getStore() =
            StoreBuilder.from(Fetcher.of { key: String ->
                Graph.provideRetrofit().fetchSubreddit(key, 10).data.children.map(Graph::toPosts)
            }).build()

}

class MyTestClass(val provider: StoreProvider) {

    suspend fun testFetching(): List<Post> {
        val store = provider.getStore()
        return store.fresh("asd")
    }

}

@ExperimentalCoroutinesApi
@FlowPreview
@RunWith(JUnit4::class)
class MyTestClassTest {
    private val testScope = TestCoroutineScope()
    private lateinit var storeProvider: StoreProvider
    private lateinit var store: Store<String, List<Post>>
    private lateinit var testClass: MyTestClass

    @Before
    fun setUp() {
        storeProvider = mock()
        store = mock()
        whenever(storeProvider.getStore()).thenReturn(store)
        testClass = MyTestClass(storeProvider)
    }


    @Test
    fun failingTest() = testScope.runBlockingTest {
        whenever(store.fresh(any())).thenReturn(mock())
        val result = testClass.testFetching()
    }
}

/*
Crashes:

java.lang.NullPointerException
	at com.dropbox.android.external.store4.StoreKt$fresh$$inlined$filterNot$1.collect(SafeCollector.common.kt:114)
	at kotlinx.coroutines.flow.FlowKt__ReduceKt.first(Reduce.kt:163)
	at kotlinx.coroutines.flow.FlowKt.first(Unknown Source)
	at com.dropbox.android.external.store4.StoreKt.fresh(Store.kt:84)
	at com.dropbox.android.sample.ui.reddit.MyTestClassTest$failingTest$1.invokeSuspend(ExampleTest.kt:70)
	at com.dropbox.android.sample.ui.reddit.MyTestClassTest$failingTest$1.invoke(ExampleTest.kt)
	at kotlinx.coroutines.test.TestBuildersKt$runBlockingTest$deferred$1.invokeSuspend(TestBuilders.kt:50)
	at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33)
	at kotlinx.coroutines.DispatchedTask.run(DispatchedTask.kt:56)
	at kotlinx.coroutines.test.TestCoroutineDispatcher.dispatch(TestCoroutineDispatcher.kt:50)
	at kotlinx.coroutines.DispatchedContinuationKt.resumeCancellableWith(DispatchedContinuation.kt:288)
	at kotlinx.coroutines.intrinsics.CancellableKt.startCoroutineCancellable(Cancellable.kt:26)
	at kotlinx.coroutines.CoroutineStart.invoke(CoroutineStart.kt:109)
	at kotlinx.coroutines.AbstractCoroutine.start(AbstractCoroutine.kt:158)
	at kotlinx.coroutines.BuildersKt__Builders_commonKt.async(Builders.common.kt:91)
	at kotlinx.coroutines.BuildersKt.async(Unknown Source)
	at kotlinx.coroutines.BuildersKt__Builders_commonKt.async$default(Builders.common.kt:84)
	at kotlinx.coroutines.BuildersKt.async$default(Unknown Source)
	at kotlinx.coroutines.test.TestBuildersKt.runBlockingTest(TestBuilders.kt:49)
	at kotlinx.coroutines.test.TestBuildersKt.runBlockingTest(TestBuilders.kt:73)
	at com.dropbox.android.sample.ui.reddit.MyTestClassTest.failingTest(ExampleTest.kt:69)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:59)
	at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
	at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:56)
	at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
	at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:26)
	at org.junit.runners.ParentRunner$3.evaluate(ParentRunner.java:306)
	at org.junit.runners.BlockJUnit4ClassRunner$1.evaluate(BlockJUnit4ClassRunner.java:100)
	at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:366)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:103)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:63)
	at org.junit.runners.ParentRunner$4.run(ParentRunner.java:331)
	at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:79)
	at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:329)
	at org.junit.runners.ParentRunner.access$100(ParentRunner.java:66)
	at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:293)
	at org.junit.runners.ParentRunner$3.evaluate(ParentRunner.java:306)
	at org.junit.runners.ParentRunner.run(ParentRunner.java:413)
	at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
	at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
	at com.intellij.rt.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:33)
	at com.intellij.rt.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:230)
	at com.intellij.rt.junit.JUnitStarter.main(JUnitStarter.java:58)

*/

I am facing the same issue, did you find the fix for the same?