Junit5 tests: replace cryptic error with more obvious one

GitOrigin-RevId: 1573da1e431dec48b89a690bf1314f8e7e094cf7

platform/testFramework/junit5/src/fixture/TestFixtureImpl.kt

@@ -1,8 +1,9 @@
-// Copyright 2000-2024 JetBrains s.r.o. and contributors. Use of this source code is governed by the Apache 2.0 license.
+// Copyright 2000-2025 JetBrains s.r.o. and contributors. Use of this source code is governed by the Apache 2.0 license.
 package com.intellij.testFramework.junit5.fixture
 
 import com.intellij.platform.util.coroutines.attachAsChildTo
 import com.intellij.platform.util.coroutines.childScope
+import com.intellij.testFramework.junit5.TestApplication
 import kotlinx.coroutines.*
 
 internal class TestFixtureImpl<T>(
@@ -19,8 +20,12 @@ internal class TestFixtureImpl<T>(
 
   override fun get(): T {
     try {
-      @Suppress("UNCHECKED_CAST")
-      val deferred = _state as Deferred<ScopedValue<T>>
+      @Suppress("UNCHECKED_CAST", "TestOnlyProblems")
+      val deferred = _state as? Deferred<ScopedValue<T>> ?: error("""
+        Fixture framework seems not be initialized. Make sure that:
+        1. A test is written in Kotlin
+        2. There is an annotation ${TestApplication::class.java.name} on top of your file. 
+      """.trimIndent())
       @OptIn(ExperimentalCoroutinesApi::class)
       return deferred.getCompleted().first
     }

platform/testFramework/junit5/test/fixture/NoAnnotationTest.kt

@@ -0,0 +1,14 @@
+// Copyright 2000-2025 JetBrains s.r.o. and contributors. Use of this source code is governed by the Apache 2.0 license.
+package com.intellij.testFramework.junit5.fixture
+
+import org.junit.jupiter.api.Assertions
+import org.junit.jupiter.api.Test
+
+class NoAnnotationTest {
+  @Test
+  fun testNoAnnotation() {
+    Assertions.assertThrows<IllegalStateException>(IllegalStateException::class.java) {
+      projectFixture().get()
+    }
+  }
+}