[DB Manager] previewing layers in Virtual layers section remove them …

…from the Layers panel: fixies #16476

plus more vlayer db_manager plugin fixes to reduce (but can't avoid) exceptions due to C++/SIP object removes

python/plugins/db_manager/db_plugins/vlayers/connector.py

@@ -96,6 +96,10 @@ def getLayer(self, l):
         lid = self.layers.get(l)
         if lid is None:
             return lid
+        # the instance can refer to a layer in map previe and not in qgis general canvas
+        if lid not in QgsMapLayerRegistry.instance().mapLayers().keys():
+            self.layers.pop(l)
+            return None
         return QgsMapLayerRegistry.instance().mapLayer(lid)
 
 
@@ -246,12 +250,16 @@ def getRasterTables(self, schema=None):
     def getTableRowCount(self, table):
         t = table[1]
         l = VLayerRegistry.instance().getLayer(t)
+        if not l or not l.isValid():
+            return None
         return l.featureCount()
 
     def getTableFields(self, table):
         """ return list of columns in table """
         t = table[1]
         l = VLayerRegistry.instance().getLayer(t)
+        if not l or not l.isValid():
+            return []
         # id, name, type, nonnull, default, pk
         n = l.dataProvider().fields().size()
         f = [(i, f.name(), f.typeName(), False, None, False)
@@ -277,6 +285,8 @@ def getTableExtent(self, table, geom):
             l = QgsMapLayerRegistry.instance().mapLayer(t)
         else:
             l = VLayerRegistry.instance().getLayer(t)
+        if not l or not l.isValid():
+            return None
         e = l.extent()
         r = (e.xMinimum(), e.yMinimum(), e.xMaximum(), e.yMaximum())
         return r

python/plugins/db_manager/layer_preview.py

@@ -25,7 +25,7 @@
 from qgis.PyQt.QtWidgets import QApplication
 
 from qgis.gui import QgsMapCanvas, QgsMapCanvasLayer, QgsMessageBar
-from qgis.core import QgsVectorLayer, QgsMapLayerRegistry
+from qgis.core import QgsVectorLayer, QgsMapLayerRegistry, QgsProject
 
 from .db_plugins.plugin import Table
 
@@ -113,15 +113,18 @@ def _loadTablePreview(self, table, limit=False):
             else:
                 vl = table.toMapLayer()
 
-            if not vl.isValid():
+            if vl and not vl.isValid():
                 vl.deleteLater()
                 vl = None
 
         # remove old layer (if any) and set new
         if self.currentLayer:
-            QgsMapLayerRegistry.instance().removeMapLayers([self.currentLayer.id()])
+            # but not remove it if in layer list panel
+            # fix https://issues.qgis.org/issues/16476
+            if not QgsProject.instance().layerTreeRoot().findLayer(self.currentLayer.id()):
+                QgsMapLayerRegistry.instance().removeMapLayers([self.currentLayer.id()])
 
-        if vl:
+        if vl and vl.isValid():
             self.setLayerSet([QgsMapCanvasLayer(vl)])
             QgsMapLayerRegistry.instance().addMapLayers([vl], False)
             self.zoomToFullExtent()