Qt C++:Signal and Slot Argument are not compatible - c++

Question

I don't know what the issue with this code, the syntax I have used for connecting the signals and sort looks correct but for some reason I keep getting this error
error: static assertion failed: Signal and slot arguments are not compatible.
# define Q_STATIC_ASSERT_X(Condition, Message) static_assert(bool(Condition), Message)
^
this is the code for the header file
include <QString>
#include <QStringList>
#include <QTextStream>
class filereader : public QObject
{
Q_OBJECT
public:
Q_PROPERTY(QString filename READ getFileName WRITE setFileName NOTIFY fileNameChanged)
Q_PROPERTY(QString output READ getOutput)
Q_PROPERTY(QString error READ getErrorString)
explicit filereader(QObject *parent = 0);
QString getFileName();
QString getOutput();
QString getErrorString();
void setFileName(const QString &fileName);
Q_INVOKABLE void readFile(); // this allows us to use this is as a function so we could do something like onClicked{ <text_id>.text = <file_reader_id>.readFile() }
signals:
void fileNameChanged();
void outputChanged(QString &Output);
void gotError(QString &&err);
void errorSignal();
protected slots:
void handleOut(QString &output);
void handleError(QString &err);
private:
QString fileName;
QString output;
QString error;
};
#endif // FILEREADER_H
this is the code for the source file
#include "filereader.h"
filereader::filereader(QObject *parent):
QObject(parent)
{
connect(this,&filereader::gotError,this,&filereader::handleError);
connect(this,&filereader::outputChanged,this,&filereader::handleOut);
}
QString filereader::getFileName()
{
return this->fileName;
}
QString filereader::getOutput()
{
return this->output;
}
QString filereader::getErrorString()
{
return this->error;
}
void filereader::setFileName(const QString &fileName)
{
if(this->fileName == fileName)
return;
this->fileName = fileName;
emit fileNameChanged();
}
void filereader::readFile()
{
QFile file(this->fileName);
if (file.open(QIODevice::ReadOnly))
{
QTextStream in(&file);
while (!in.atEnd())
{
QString line = in.readAll();
outputChanged(line);
qDebug() << line;
}
file.close();
file.flush(); // this flushes the file stream
}else{
gotError("Could not open the file!");
}
}
void filereader::handleOut(QString &output)
{
if(this->output == output)
return;
this->output = output;
}
void filereader::handleError(QString &err)
{
if(this->error == err)
return;
this->error = err;
emit errorSignal();
}
I'm new to Qt and Qml so please help me understand the issue. Thanks :)