Commit all documents after J2K (KT-13584, EA-82451)

New Kotlin file will be generated after converting, they are created for virtual files that were renamed from Java.
The exception happens when Java file was previously opened in editor, in that case text is restored through getLastCommittedText(document) where
document is resored for virtual file.

See: SingleRootFileViewProvider.VirtualFileContent.getText()

Exception descriptions also mention "Create Test" action as a problem source. The issue was reproduced when J2K convertion was performed
on test creation.

(cherry picked from commit e44260a)

 #KT-13584 Fixed

idea/src/org/jetbrains/kotlin/idea/actions/JavaToKotlinAction.kt

@@ -89,7 +89,7 @@ class JavaToKotlinAction : AnAction() {
                     }
                 }
                 catch (e: IOException) {
-                    MessagesEx.error(psiFile.project, e.message).showLater()
+                    MessagesEx.error(psiFile.project, e.message ?: "").showLater()
                 }
             }
             return result
@@ -138,6 +138,8 @@ class JavaToKotlinAction : AnAction() {
 
                 externalCodeUpdate?.invoke()
 
+                PsiDocumentManager.getInstance(project).commitAllDocuments()
+
                 newFiles.singleOrNull()?.let {
                     FileEditorManager.getInstance(project).openFile(it, true)
                 }