IntelliJ plugin development. Insert a string in a file

Code:

public class JavaCodePredictionProvider extends CompletionProvider<CompletionParameters> {

  @Override
  protected void addCompletions(@NotNull CompletionParameters parameters,
      @NotNull ProcessingContext context, @NotNull CompletionResultSet result) {

    Editor editor = parameters.getEditor();
    Project project = parameters.getEditor().getProject();
    PsiFile psiFile = PsiDocumentManager.getInstance(project).getPsiFile(editor.getDocument());
    prepare(editor, psiFile, project);
  }

 public void prepare(Editor editor, PsiFile psiFile, Project project) {

  // ...code omitted...

  String codeSuggestion = "This is the string that I would like to insert into the file";
  suggest(project, editor, codeSuggestion, offset);
 }

  // this method displays the suggestion to the user
  public void suggest(Project project, Editor editor, String codeSuggestion, int offset) {
    // Get the document and project
    final Document document = editor.getDocument();
    // Construct the runnable to substitute the string at offset in the document
    Runnable runnable = () -> document.insertString(offset, codeSuggestion);
    // Make the document change in the context of a write action.
    WriteCommandAction.runWriteCommandAction(project, runnable);
  }

}

I am developing  a plugin for IntelliJ. 

This plugin should return a code suggestion. I don't want the suggestion to appear in the list among the predictions already provided by IntelliJ, however I would like my prediction to appear in a light grey color in the file and then the user can choose to accept it or not (through a shortcut).

In my suggest method I am trying to inject the codeSuggestion string into the file. However I am getting an error saying:

"Must not start write action from within read action in the other thread - deadlock is coming
java.lang.Throwable: Must not start write action from within read action in the other thread - deadlock is coming"

Are there other ways to insert a string in a file? Maybe by modifying the PSI?

Thank you