PyQGIS using selected layer from a combobox

There is a recommended way to do this: Use the built-in custom widget called QgsMapLayerComboBox.

You can find it in Qt Designer under the QGIS custom widgets group.

Once you open the dialog in QGIS, the combo box will be filled with all the layers in the QgsProject. QgsMapLayerComboBox offers handy methods for you to filter such list (see the docs).

When you're ready to read and use the selected layer, e.g., after the users accepts the dialog, you can access the selected layer in this way:

layer = self.dlg.mMapLayerComboBox.currentLayer()  # Real QgsMapLayer object

And that's it!

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Annex: Why is your code failing?

Using a normal Qt ComboBox, you have several options to access a layer selected in the combo box. The best way is to use my_combo_box.addItem(layer_name, layer_object) instead of using my_combo_box.addItems(...) for filling the combo box.

In this way you can then get direct access to the selected layer using

layer = self.dlg.comboBox.currentData()

You can access the current text in your QComboBox via

lyr_name = my_combo.currentText()

and the layer via

the_layer = QgsProject.instance().mapLayersByName(lyr_name)[0]

mapLayersByName() method returns a list because QGIS can handle various layers of the same name. Thus, I often use [0] because in most of my cases the layer names are unique.

Reference: https://qgis.org/api/classQgsProject.html#a935efacacb942a15afe19728233e35c3

I recommend the answer of Germán Carrillo. I'm adding another way to get these custom widgets, just in case if it's not listed in QtDesigner, in the left menu.

You can use a normal QComboBox in QtDesigner. Then right-clic on it and choose Promote to....

Then you can add custom widgets:

Name of base class: `QComboBox`
Name of promoted class: `QgsMapLayerComboBox`
Header: `qgis.gui`