Merge pull request #4533 from boundlessgeo/too_many_dbmanager_errors_…

…fix#15868

[db_manager] catch exception when item has lost reference. for 2.18 regression fixes #15868

python/plugins/db_manager/db_tree.py

@@ -164,7 +164,7 @@ def addLayer(self):
             layers = QgsMapLayerRegistry.instance().addMapLayers([layer])
             if len(layers) != 1:
                 QgsMessageLog.logMessage(
-                    self.tr("%1 is an invalid layer - not loaded").replace("%1", layer.publicSource()))
+                    self.tr("%1 is an invalid layer - not loaded").replace("%1", layer.publicSource()), "DBManagerPlugin")
                 msgLabel = QLabel(self.tr(
                     "%1 is an invalid layer and cannot be loaded. Please check the <a href=\"#messageLog\">message log</a> for further info.").replace(
                     "%1", layer.publicSource()), self.mainWindow.infoBar)

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, QgsProject
+from qgis.core import QgsVectorLayer, QgsMapLayerRegistry, QgsProject, QgsMessageLog
 
 from .db_plugins.plugin import Table
 
@@ -78,11 +78,13 @@ def setDirty(self, val=True):
     def _clear(self):
         """ remove any layers from preview canvas """
         if self.item is not None:
-            ## skip exception on RuntimeError fixes #6892
             try:
                 self.item.aboutToChange.disconnect(self.setDirty)
-            except RuntimeError:
-                pass
+            ## skip exception on RuntimeError fixes #6892
+            ## skip TypeError and generic Exceptions fixes #15868
+            ## generally due the remove of self.item object or C++ referenced object
+            except Exception as ex:
+                QgsMessageLog.logMessage(unicode(ex), "DBManagerPlugin")
 
         self.item = None
         self.dirty = False