Check if webview exists

app/src/main/java/eu/faircode/email/FragmentMessage.java

@@ -557,6 +557,7 @@ public class FragmentMessage extends FragmentEx {
         menu.findItem(R.id.menu_show_headers).setChecked(headers);
         menu.findItem(R.id.menu_show_headers).setEnabled(message.uid != null);
         menu.findItem(R.id.menu_show_headers).setVisible(!free);
+        menu.findItem(R.id.menu_show_html).setEnabled(Helper.classExists("android.webkit.WebView"));
         menu.findItem(R.id.menu_reply_all).setVisible(!inOutbox);
     }
 

app/src/main/java/eu/faircode/email/Helper.java

@@ -124,6 +124,15 @@ public class Helper {
         return String.format("%.1f %sB", bytes / Math.pow(unit, exp), pre);
     }
 
+    static boolean classExists(String className) {
+        try {
+            Class.forName(className);
+            return true;
+        } catch (ClassNotFoundException ignored) {
+            return false;
+        }
+    }
+
     static Address myAddress() throws UnsupportedEncodingException {
         return new InternetAddress("marcel+fairemail@faircode.eu", "FairCode");
     }