Database manager cannot import layer if any layer is None

[sickel]

Describe the bug

Trying to use db manager to import a layer to a postgis database by pressing "Import layer/file" when a postgis database is connected, a python error is displayed and the available layers are not being listed. This happens if there is a defective layer in the project from the Open Layers plugin.

How to Reproduce

Make a connection to a postgis database, Install the open layer plugin. In a project with CRS EPSG:3857 Pseudo mercator add in e.g. Bing Roads from open layers. Save and close the project and reopen, the layer will not display. Open db manager, connect to the postgis database and press "Import layer/file". A python error will be displayed.

QGIS and OS versions
Last tested on qgis 3.16-3 on windows and linux (debian), but also seen on several previous versions.

Additional context

The bug is triggerd by a bug in open layers, but as far as I can see, the fix will not have any negative concequences and will generally make the import function more robust.

** Proposed fix **

The error message is (on my Windows 10 PC)
File "C:/PROGRA1/QGIS31.16/apps/qgis/./python/plugins\db_manager\dlg_import_vector.py", line 135, in populateLayers
if layer.type() == QgsMapLayerType.VectorLayer:
AttributeError: 'NoneType' object has no attribute 'type'

The present code is:
def populateLayers(self):
self.cboInputLayer.clear()
for nodeLayer in QgsProject.instance().layerTreeRoot().findLayers():
layer = nodeLayer.layer()
# TODO: add import raster support!
if layer.type() == QgsMapLayerType.VectorLayer:
self.cboInputLayer.addItem(layer.name(), layer.id())

this could be rewritten as:

def populateLayers(self):
    self.cboInputLayer.clear()
    for nodeLayer in QgsProject.instance().layerTreeRoot().findLayers():
        layer = nodeLayer.layer()
        # TODO: add import raster support!
        if layer != None:
            if layer.type() == QgsMapLayerType.VectorLayer:
                self.cboInputLayer.addItem(layer.name(), layer.id())

It could of course also be done by a try / expect or a check if layer is the right type of object.

[gioman]

** Proposed fix **

The error message is (on my Windows 10 PC)
File "C:/PROGRA1/QGIS31.16/apps/qgis/./python/plugins\db_manager\dlg_import_vector.py", line 135, in populateLayers
if layer.type() == QgsMapLayerType.VectorLayer:
AttributeError: 'NoneType' object has no attribute 'type'

The present code is:
def populateLayers(self):
self.cboInputLayer.clear()
for nodeLayer in QgsProject.instance().layerTreeRoot().findLayers():
layer = nodeLayer.layer()

TODO: add import raster support!

if layer.type() == QgsMapLayerType.VectorLayer:
self.cboInputLayer.addItem(layer.name(), layer.id())

this could be rewritten as:

def populateLayers(self):
    self.cboInputLayer.clear()
    for nodeLayer in QgsProject.instance().layerTreeRoot().findLayers():
        layer = nodeLayer.layer()
        # TODO: add import raster support!
        if layer != None:
            if layer.type() == QgsMapLayerType.VectorLayer:
                self.cboInputLayer.addItem(layer.name(), layer.id())

It could of course also be done by a try / expect or a check if layer is the right type of object.

@sickel please make in the form of a pull request, thanks.