Defer calling postProcessors until layers have been added to layer tree

It's possible that an algorithm's postProcessor may contain logic
relating to the layer tree, so make sure that all layers have
already been added to the tree before calling postProcessors

python/plugins/processing/gui/Postprocessing.py

@@ -198,6 +198,8 @@ def handleAlgorithmResults(alg: QgsProcessingAlgorithm,
     i = 0
 
     added_layers: List[Tuple[QgsLayerTreeGroup, QgsLayerTreeLayer]] = []
+    layers_to_post_process: List[Tuple[QgsMapLayer,
+                                       QgsProcessingContext.LayerDetails]] = []
 
     for dest_id, details in context.layersToLoadOnCompletion().items():
         if feedback.isCanceled():
@@ -235,10 +237,11 @@ def handleAlgorithmResults(alg: QgsProcessingAlgorithm,
                 added_layers.append((results_group, layer_tree_layer))
 
                 if details.postProcessor():
-                    details.postProcessor().postProcessLayer(
-                        layer,
-                        context,
-                        feedback)
+                    # we defer calling the postProcessor set in the context
+                    # until the layer has been added to the project's layer
+                    # tree, just in case the postProcessor contains logic
+                    # relating to layer tree handling
+                    layers_to_post_process.append((layer, details))
 
             else:
                 wrong_layers.append(str(dest_id))
@@ -262,6 +265,14 @@ def handleAlgorithmResults(alg: QgsProcessingAlgorithm,
         layer_node.removeCustomProperty(SORT_ORDER_CUSTOM_PROPERTY)
         group.insertChildNode(0, layer_node)
 
+    # all layers have been added to the layer tree, so safe to call
+    # postProcessors now
+    for layer, details in layers_to_post_process:
+        details.postProcessor().postProcessLayer(
+            layer,
+            context,
+            feedback)
+
     feedback.setProgress(100)
 
     if wrong_layers: