Check that a file with .kt extension is actually a KtFile

#KT-22111 Fixed

idea/idea-analysis/src/org/jetbrains/kotlin/idea/vfilefinder/KotlinPackageSourcesMemberNamesIndex.kt

@@ -1,17 +1,6 @@
 /*
- * Copyright 2010-2017 JetBrains s.r.o.
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- * http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
+ * Copyright 2000-2018 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license
+ * that can be found in the license/LICENSE.txt file.
  */
 
 package org.jetbrains.kotlin.idea.vfilefinder
@@ -45,7 +34,7 @@ object KotlinPackageSourcesMemberNamesIndex : FileBasedIndexExtension<String, Co
 
     override fun getIndexer(): DataIndexer<String, Collection<String>, FileContent> =
         DataIndexer { inputData ->
-            val ktFile = inputData.psiFile as KtFile
+            val ktFile = inputData.psiFile as? KtFile ?: return@DataIndexer emptyMap()
             val packageName = ktFile.packageDirective?.fqName?.asString() ?: ""
 
             mapOf(packageName to ktFile.declarations.mapNotNullTo(hashSetOf(), KtDeclaration::getName))