#include "importdicombrowser.h"

void qSlicerMainWindow::on_LoadAction_triggered()
{
  ImportDICOMBrowser* temp = new ImportDICOMBrowser(this);
  temp->show();
}

• ImportDICOMBrowser.h

#ifndef IMPORTDICOMBROWSER_H
#define IMPORTDICOMBROWSER_H

#include <QDialog>

namespace Ui {
class ImportDICOMBrowser;
}

class ImportDICOMBrowser : public QDialog
{
    Q_OBJECT

public:
    explicit ImportDICOMBrowser(QWidget *parent = nullptr);
    ~ImportDICOMBrowser();

private:
    Ui::ImportDICOMBrowser *ui;
};

#endif // IMPORTDICOMBROWSER_H

• ImportDICOMBrowser.cxx

#include "importdicombrowser.h"
#include "ui_importdicombrowser.h"

ImportDICOMBrowser::ImportDICOMBrowser(QWidget *parent) :
    QDialog(parent),
    ui(new Ui::ImportDICOMBrowser)
{
    ui->setupUi(this);
}

ImportDICOMBrowser::~ImportDICOMBrowser()
{
    delete ui;
}

• ERROR:

Error	LNK2019	unresolved external symbol "public: __cdecl ImportDICOMBrowser::ImportDICOMBrowser(class QWidget *)" (??0ImportDICOMBrowser@@QEAA@PEAVQWidget@@@Z) referenced in function "public: void __cdecl qSlicerMainWindow::on_LoadAction_triggered(void)" (?on_LoadAction_triggered@qSlicerMainWindow@@QEAAXXZ) 

• I am not able to understand why is it showing ImportDICOMBrowser as an external variable, I also included the hearder file #include "importdicombrowser.h"