Cannot link to custom Qt widget - c++

Question

I'm new to Qt and am trying to figure out how to add a custom widget I've created to my main window.
The widget is pretty simple - I just used Qt Designer to create the default widget (I'm calling it Editor) and then added a button to it. Next I included the header of my custom widget in mainwindow.h and added a pointer to it in my MainWindow class. So far, so good. But when I try to create a new instance of Editor, I get the error:
mainwindow.obj:-1: error: LNK2019: unresolved external symbol "public: __cdecl Editor::Editor(class QWidget *)" (??0Editor##QEAA#PEAVQWidget###Z) referenced in function "public: __cdecl MainWindow::MainWindow(class QWidget *)" (??0MainWindow##QEAA#PEAVQWidget###Z)
What do I need to do to be able to link my code?
mainwindow.h:
#ifndef MAINWINDOW_H
#define MAINWINDOW_H
#include <QMainWindow>
#include "editor.h"
namespace Ui {
class MainWindow;
}
class MainWindow : public QMainWindow
{
Q_OBJECT
public:
explicit MainWindow(QWidget *parent = 0);
~MainWindow();
private:
Ui::MainWindow *ui;
Editor *editor;
};
#endif // MAINWINDOW_H
mainwindow.cpp:
#include "mainwindow.h"
#include "ui_mainwindow.h"
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
editor = new Editor(0);
}
MainWindow::~MainWindow()
{
delete ui;
delete editor;
}
editor.h:
#ifndef EDITOR_H
#define EDITOR_H
#include <QWidget>
namespace Ui {
class Editor;
}
class Editor : public QWidget
{
Q_OBJECT
public:
explicit Editor(QWidget *parent = 0);
~Editor();
private:
Ui::Editor *ui;
};
#endif // EDITOR_H
editor.cpp:
#include "editor.h"
#include "ui_editor.h"
Editor::Editor(QWidget *parent) :
QWidget(parent),
ui(new Ui::Editor)
{
ui->setupUi(this);
}
Editor::~Editor()
{
delete ui;
}
CustomWidgetTest.pro:
QT += core gui
greaterThan(QT_MAJOR_VERSION, 4): QT += widgets
TARGET = CustomWidgetTest
TEMPLATE = app
SOURCES += main.cpp\
mainwindow.cpp \
editor.cpp
HEADERS += mainwindow.h \
editor.h
FORMS += mainwindow.ui \
editor.ui