title: android.view.View$OnUnhandledKeyEventListener
body: <p>I am new to Android Studio and I don't get why my toolbar isn't shown as described by <a href="https://developer.android.com/training/appbar/setting-up" rel="noreferrer">https://developer.android.com/training/appbar/setting-up</a>
  I know there are already some other questions like mine on stackoverflow but they don't work at my project. Therefore I would be very thankful for fixing this issue. Screenshot:</p>
  
  <p><a href="https://i.stack.imgur.com/kpx3o.png" rel="noreferrer"><img src="https://i.stack.imgur.com/kpx3o.png" alt="Screenshot"></a></p>
  
  <pre><code>java.lang.ClassNotFoundException: android.view.View$OnUnhandledKeyEventListener
  at org.jetbrains.android.uipreview.ModuleClassLoader.load(ModuleClassLoader.java:180)
  at com.android.tools.idea.rendering.RenderClassLoader.findClass(RenderClassLoader.java:61)
  at org.jetbrains.android.uipreview.ModuleClassLoader.findClass(ModuleClassLoader.java:118)
  at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
  at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
  at org.jetbrains.android.uipreview.ModuleClassLoader.loadClass(ModuleClassLoader.java:213)
  at android.support.v7.widget.ViewUtils.isLayoutRtl(ViewUtils.java:58)
  at android.support.v7.widget.Toolbar.onMeasure_Original(Toolbar.java:1578)
  at android.support.v7.widget.Toolbar.onMeasure(Toolbar.java)
  at android.view.View.measure_Original(View.java:22071)
  at android.view.View_Delegate.measure(View_Delegate.java:80)
  at android.view.View.measure(View.java:22035)
  at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6602)
  at android.widget.FrameLayout.onMeasure(FrameLayout.java:185)
  at android.view.View.measure_Original(View.java:22071)
  at android.view.View_Delegate.measure(View_Delegate.java:80)
  at android.view.View.measure(View.java:22035)
  at android.widget.RelativeLayout.measureChildHorizontal(RelativeLayout.java:715)
  at android.widget.RelativeLayout.onMeasure(RelativeLayout.java:461)
  at android.view.View.measure_Original(View.java:22071)
  at android.view.View_Delegate.measure(View_Delegate.java:80)
  at android.view.View.measure(View.java:22035)
  at com.android.layoutlib.bridge.impl.RenderSessionImpl.measureView(RenderSessionImpl.java:590)
  at com.android.layoutlib.bridge.impl.RenderSessionImpl.inflate(RenderSessionImpl.java:343)
  at com.android.layoutlib.bridge.Bridge.createSession(Bridge.java:384)
  at com.android.tools.idea.layoutlib.LayoutLibrary.createSession(LayoutLibrary.java:193)
  at com.android.tools.idea.rendering.RenderTask.createRenderSession(RenderTask.java:544)
  at com.android.tools.idea.rendering.RenderTask.lambda$inflate$3(RenderTask.java:678)
  at java.util.concurrent.FutureTask.run(FutureTask.java:266)
  at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
  at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
  at java.lang.Thread.run(Thread.java:745)
  </code></pre>
  
  <h1>toolbar.xml</h1>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
  &lt;android.support.v7.widget.Toolbar xmlns:android="http://schemas.android.com/apk/res/android"
      android:layout_width="match_parent"
      android:layout_height="match_parent"
      android:background="@color/colorPrimaryDark"&gt;
  
  &lt;/android.support.v7.widget.Toolbar&gt;
  </code></pre>
  
  <h1>build.gradle (app level)</h1>
  
  <pre><code>apply plugin: 'com.android.application'
  android {
      compileSdkVersion 28
      defaultConfig {
          applicationId "com.example.david.gamebase"
          minSdkVersion 21
          targetSdkVersion 28
          versionCode 1
          versionName "1.0"
          testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
      }
      buildTypes {
          release {
              minifyEnabled false
              proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
          }
      }
  }
  
  dependencies {
      implementation fileTree(dir: 'libs', include: ['*.jar'])
      implementation 'com.android.support:appcompat-v7:28.0.0-rc01'
      implementation 'com.android.support.constraint:constraint-layout:1.1.2'
      implementation 'com.google.firebase:firebase-auth:11.6.0'
      testImplementation 'junit:junit:4.12'
      androidTestImplementation 'com.android.support.test:runner:1.0.2'
      androidTestImplementation 'com.android.support.test.espresso:espresso-core:3.0.2'
  }
  
  apply plugin: 'com.google.gms.google-services'
  </code></pre>
  
  <h2>style.xml:</h2>
  
  <pre><code>   &lt;resources&gt;
  
      &lt;!-- Base application theme. --&gt;
      &lt;style name="AppTheme" parent="Base.Theme.AppCompat.Light.DarkActionBar"&gt;
          &lt;!-- Customize your theme here. --&gt;
          &lt;item name="colorPrimary"&gt;@color/colorPrimary&lt;/item&gt;
          &lt;item name="colorPrimaryDark"&gt;@color/colorPrimaryDark&lt;/item&gt;
          &lt;item name="colorAccent"&gt;@color/colorAccent&lt;/item&gt;
      &lt;/style&gt;
  
  &lt;/resources&gt;
  
  ## toolbar.xml ##
  
      &lt;?xml version="1.0" encoding="utf-8"?&gt;
  &lt;android.support.v7.widget.Toolbar xmlns:android="http://schemas.android.com/apk/res/android"
      android:layout_width="match_parent"
      android:layout_height="match_parent"
      android:background="@color/colorPrimaryDark"&gt;
  
  &lt;/android.support.v7.widget.Toolbar&gt;
  </code></pre>