Copy: Fix exception on copying KtFile to non-KtFile

#KT-22669 Fixed

idea/src/org/jetbrains/kotlin/idea/refactoring/copy/CopyKotlinDeclarationsHandler.kt

@@ -257,18 +257,20 @@ class CopyKotlinDeclarationsHandler : CopyHandlerDelegateBase() {
                         else -> null
                     }
 
-                    val targetFile: KtFile
+                    val targetFile: PsiFile
                     val copiedDeclaration: KtNamedDeclaration?
                     if (fileToCopy != null) {
                         targetFile = runWriteAction {
-                            val copiedFile = targetDirectory.copyFileFrom(targetFileName, fileToCopy) as KtFile
-                            if (fileToCopy.packageMatchesDirectory()) {
+                            val copiedFile = targetDirectory.copyFileFrom(targetFileName, fileToCopy)
+                            if (copiedFile is KtFile && fileToCopy.packageMatchesDirectory()) {
                                 targetDirectory.getPackage()?.qualifiedName?.let { copiedFile.packageFqName = FqName(it) }
                             }
                             performDelayedRefactoringRequests(project)
                             copiedFile
                         }
-                        copiedDeclaration = if (isSingleDeclarationInFile) targetFile.declarations.singleOrNull() as? KtNamedDeclaration else null
+                        copiedDeclaration = if (isSingleDeclarationInFile && targetFile is KtFile) {
+                            targetFile.declarations.singleOrNull() as? KtNamedDeclaration
+                        } else null
                     }
                     else {
                         targetFile = getOrCreateTargetFile(originalFile, targetDirectory, targetFileName, commandName) ?: return@executeCommand

idea/testData/refactoring/copy/copyKtFileToTextFile/after/foo/test.kt

@@ -0,0 +1,8 @@
+package foo
+
+val a = 42
+
+// comment
+
+// some other comment
+val s = ""

idea/testData/refactoring/copy/copyKtFileToTextFile/after/foo/test.kt.txt

@@ -0,0 +1,8 @@
+package foo
+
+val a = 42
+
+// comment
+
+// some other comment
+val s = ""

idea/testData/refactoring/copy/copyKtFileToTextFile/before/foo/test.kt

@@ -0,0 +1,8 @@
+package foo
+
+val a = 42
+
+// comment
+
+// some other comment
+val s = ""

idea/testData/refactoring/copy/copyKtFileToTextFile/copyKtFileToTextFile.test

@@ -0,0 +1,5 @@
+{
+  "mainFile": "foo/test.kt",
+  "targetPackage": "foo",
+  "newName": "test.kt.txt"
+}

idea/tests/org/jetbrains/kotlin/idea/refactoring/copy/CopyTestGenerated.java

@@ -91,6 +91,12 @@ public class CopyTestGenerated extends AbstractCopyTest {
         doTest(fileName);
     }
 
+    @TestMetadata("copyKtFileToTextFile/copyKtFileToTextFile.test")
+    public void testCopyKtFileToTextFile_CopyKtFileToTextFile() throws Exception {
+        String fileName = KotlinTestUtils.navigationMetadata("idea/testData/refactoring/copy/copyKtFileToTextFile/copyKtFileToTextFile.test");
+        doTest(fileName);
+    }
+
     @TestMetadata("copyLocalClass/copyLocalClass.test")
     public void testCopyLocalClass_CopyLocalClass() throws Exception {
         String fileName = KotlinTestUtils.navigationMetadata("idea/testData/refactoring/copy/copyLocalClass/copyLocalClass.test");