make text box stay open

Co-authored-by: Paul Romano <paul.k.romano@gmail.com>

Author: kkiesling

openmc_plotter/main_window.py

@@ -1247,6 +1247,7 @@ def viewMaterialProps(self, id):
         for nuc, dens in zip(mat.nuclides, mat.densities):
             msg_str += f'{nuc}: {dens:5.3e}\n'
 
-        msg_box = QMessageBox()
+        msg_box = QMessageBox(self)
         msg_box.setText(msg_str)
-        msg_box.exec_()
+        msg_box.setModal(False)
+        msg_box.show()