[processing] Fix crashes and random behavior after dropping algorithms

to model designer

Also fixes drag and drop within the algorithm parameter dialog and
the qt warnings thrown during these operations

(cherry picked from commit 30f786c)

python/plugins/processing/modeler/ModelerDialog.py

@@ -46,7 +46,8 @@
     pyqtSignal,
     QDataStream,
     QIODevice,
-    QUrl)
+    QUrl,
+    QTimer)
 from qgis.PyQt.QtWidgets import (QGraphicsView,
                                  QTreeWidget,
                                  QMessageBox,
@@ -321,19 +322,26 @@ def _dragEnterEvent(event):
                 event.ignore()
 
         def _dropEvent(event):
-            if event.mimeData().hasFormat('application/x-vnd.qgis.qgis.algorithmid'):
-                data = event.mimeData().data('application/x-vnd.qgis.qgis.algorithmid')
-                stream = QDataStream(data, QIODevice.ReadOnly)
-                algorithm_id = stream.readQString()
+            def alg_dropped(algorithm_id, pos):
                 alg = QgsApplication.processingRegistry().createAlgorithmById(algorithm_id)
                 if alg is not None:
-                    self._addAlgorithm(alg, event.pos())
+                    self._addAlgorithm(alg, pos)
                 else:
                     assert False, algorithm_id
+
+            def input_dropped(id, pos):
+                if id in [param.id() for param in QgsApplication.instance().processingRegistry().parameterTypes()]:
+                    self.addInputOfType(itemId, pos)
+
+            if event.mimeData().hasFormat('application/x-vnd.qgis.qgis.algorithmid'):
+                data = event.mimeData().data('application/x-vnd.qgis.qgis.algorithmid')
+                stream = QDataStream(data, QIODevice.ReadOnly)
+                algorithm_id = stream.readQString()
+                QTimer.singleShot(0, lambda id=algorithm_id, pos=event.pos(): alg_dropped(id, pos))
+                event.accept()
             elif event.mimeData().hasText():
                 itemId = event.mimeData().text()
-                if itemId in [param.id() for param in QgsApplication.instance().processingRegistry().parameterTypes()]:
-                    self.addInputOfType(itemId, event.pos())
+                QTimer.singleShot(0, lambda id=itemId, pos=event.pos(): input_dropped(id, pos))
                 event.accept()
             else:
                 event.ignore()