[processing] Don't abort when missing field name specified in

delete columns algorithm

Fixes #19256

(cherry-picked from 85fba79)

python/plugins/processing/algs/qgis/DeleteColumn.py

@@ -68,13 +68,22 @@ def outputName(self):
 
     def prepareAlgorithm(self, parameters, context, feedback):
         self.fields_to_delete = self.parameterAsFields(parameters, self.COLUMNS, context)
-        return True
+
+        source = self.parameterAsSource(parameters, 'INPUT', context)
+        if source is not None:
+            for f in self.fields_to_delete:
+                index = source.fields().lookupField(f)
+                if index < 0:
+                    feedback.pushInfo(self.tr('Field “{}” does not exist in input layer').format(f))
+
+        return super().prepareAlgorithm(parameters, context, feedback)
 
     def outputFields(self, input_fields):
         # loop through twice - first we need to build up a list of original attribute indices
         for f in self.fields_to_delete:
             index = input_fields.lookupField(f)
-            self.field_indices.append(index)
+            if index >= 0:
+                self.field_indices.append(index)
 
         # important - make sure we remove from the end so we aren't changing used indices as we go
         self.field_indices.sort(reverse=True)

python/plugins/processing/tests/testdata/qgis_algorithm_tests.yaml

@@ -2143,7 +2143,7 @@ tests:
   - algorithm: qgis:deletecolumn
     name: Delete columns (multiple)
     params:
-      COLUMN: floatval;name
+      COLUMN: floatval;name;xxxxxx
       INPUT:
         name: polys.gml
         type: vector