Fix null reference in ConnectionWizardWindow.

Disable Connect button until the database driver is selected.

src/AddIns/DisplayBindings/Data/ICSharpCode.Data.Core.UI/Windows/ConnectionWizardWindow.xaml

@@ -72,7 +72,14 @@
         
         <ContentControl Grid.Row="3" Grid.Column="1" Margin="3" Content="{Binding SelectedDatasource.ControlPanel, ElementName=This}" />
         
-        <Button x:Name="btnConnect" Grid.Row="4" Grid.Column="1" Margin="3" Content="Connect" HorizontalAlignment="Right" Click="btnConnect_Click" />
+        <Button x:Name="btnConnect"
+                Grid.Row="4"
+                Grid.Column="1"
+                Margin="3"
+                Content="Connect"
+                HorizontalAlignment="Right"
+                IsEnabled="{Binding CanConnect, ElementName=This}"
+                Click="btnConnect_Click" />
         
         <TextBlock Grid.Row="5" Margin="3">Database:</TextBlock>
 

src/AddIns/DisplayBindings/Data/ICSharpCode.Data.Core.UI/Windows/ConnectionWizardWindow.xaml.cs

@@ -58,9 +58,14 @@ namespace ICSharpCode.Data.Core.UI.Windows
 				_selectedDatabaseDriver = value;
 				OnPropertyChanged("SelectedDatabaseDriver");
 				OnPropertyChanged("SelectedDatasource");
+				OnPropertyChanged("CanConnect");
 			}
 		}
 		
+		public bool CanConnect {
+			get { return _selectedDatabaseDriver != null; }
+		}
+		
 		IDatasource GetDefaultDatasource(IDatabaseDriver driver)
 		{
 			if (driver != null) {
@@ -77,6 +82,7 @@ namespace ICSharpCode.Data.Core.UI.Windows
 				else
 					_selectedDatasource = defaultDataSource;
 				OnPropertyChanged("SelectedDatasource");
+				OnPropertyChanged("CanConnect");
 			}
 		}