Add error message for zero-point case.

main_window.py

@@ -2,7 +2,7 @@ from functools import partial
 
 from PyQt5.QtCore import Qt
 from PyQt5.QtGui import QCursor
-from PyQt5.QtWidgets import QFileDialog, QInputDialog, QMainWindow
+from PyQt5.QtWidgets import QErrorMessage, QFileDialog, QInputDialog, QMainWindow
 
 from clusterview2.exceptions import handle_exceptions
 from clusterview2.colors import Color
@@ -167,6 +167,12 @@ class MainWindow(QMainWindow, Ui_MainWindow):
         self.opengl_widget.update()
 
     def _clustering(self):
+        if len(list(PointManager.point_set.points)) == 0:
+            error_dialog = QErrorMessage()
+            error_dialog.showMessage('Place points before clustering.')
+            error_dialog.exec_()
+            return
+
         clear_selection()
         self._mode = Mode.CLUSTERING
         self.opengl_widget.setCursor(QCursor(Qt.CursorShape.ArrowCursor))