Improve exception if a (multi)selection prompt is used incorrectly

Before this commit, the selection prompt would throw an `InvalidOperationException` (Sequence contains no elements) and the multi selection prompt would throw an `ArgumentOutOfRangeException` (Index was out of range. Must be non-negative and less than the size of the collection.)

Both would occur because the prompts were never meant to be empty.

src/Spectre.Console/Prompts/List/ListPrompt.cs

@@ -42,6 +42,11 @@ public async Task<ListPromptState<T>> Show(
         }
 
         var nodes = tree.Traverse().ToList();
+        if (nodes.Count == 0)
+        {
+            throw new InvalidOperationException("Cannot show an empty selection prompt. Please call the AddChoice() method to configure the prompt.");
+        }
+
         var state = new ListPromptState<T>(nodes, converter, _strategy.CalculatePageSize(_console, nodes.Count, requestedPageSize), wrapAround, selectionMode, skipUnselectableItems, searchEnabled);
         var hook = new ListPromptRenderHook<T>(_console, () => BuildRenderable(state));
 

src/Tests/Spectre.Console.Tests/Unit/Prompts/MultiSelectionPromptTests.cs

@@ -127,6 +127,23 @@ public void Should_Throw_When_Getting_Parents_Of_Non_Existing_Node()
         // Then
         action.ShouldThrow<ArgumentOutOfRangeException>();
     }
+
+    [Fact] public void Should_Throw_Meaningful_Exception_For_Empty_Prompt()
+    {
+        // Given
+        var console = new TestConsole();
+        console.Profile.Capabilities.Interactive = true;
+        console.Input.PushKey(ConsoleKey.Spacebar);
+
+        var prompt = new MultiSelectionPrompt<string>();
+
+        // When
+        Action action = () => prompt.Show(console);
+
+        // Then
+        var exception = action.ShouldThrow<InvalidOperationException>();
+        exception.Message.ShouldBe("Cannot show an empty selection prompt. Please call the AddChoice() method to configure the prompt.");
+    }
 }
 
 file sealed class CustomItem

src/Tests/Spectre.Console.Tests/Unit/Prompts/SelectionPromptTests.cs

@@ -114,6 +114,22 @@ public void Should_Search_In_Remapped_Result()
         // Then
         selection.ShouldBe(choices[1]);
     }
+
+    [Fact] public void Should_Throw_Meaningful_Exception_For_Empty_Prompt()
+    {
+        // Given
+        var console = new TestConsole();
+        console.Profile.Capabilities.Interactive = true;
+
+        var prompt = new SelectionPrompt<string>();
+
+        // When
+        Action action = () => prompt.Show(console);
+
+        // Then
+        var exception = action.ShouldThrow<InvalidOperationException>();
+        exception.Message.ShouldBe("Cannot show an empty selection prompt. Please call the AddChoice() method to configure the prompt.");
+    }
 }
 
 file sealed class CustomSelectionItem