JNI calling Java from C++ with multiple threads

I'm working on a project, where I call Java functions from C++ code (using JNI) and I have a problem about multithreading. I want to call Java searching function and for each call I want to make a separate thread. I have a singleton MainClass and nested Query class. Query class is inherited from QThread. Code looks like this

MainClass::MyQuery query("<some search query>");
query.LaunchQuery();


//functions of Query   
void MainClass::MyQuery::LaunchQuery() const
{
    this->start();
}

void MainClass::Query::run()
{
    const MainClass& mainClass = MainClass::GetInstance();
    const jclass& obj = mainClass.GetClass();
    JNIEnv& env = mainClass.GetJavaEnvironment();
    jmethodID methodId = env.GetMethodID(obj, "SearchQuery", "(Ljava/lang/String;)V"); //Run-time error

    if(methodId != 0)
    {
        //calling "SearchQuery" function
    }

Now, if run this code in a single thread - everything is fine, but if try to run above code - using multithreading, it causes run-time error by message "Unhandled exception at 0x777715de in MyApp.exe: 0xC0000005: Access violation reading location 0x000000ac." when I try to get method id. I've tried also with boost::thread but result was the same.

So why it fails when I'm doing it in a separate thread, when in the same thread everything is fine? Any ideas?