Add defensive code when root project is not defined

And add a test for this.

For eng/ide/ada_language_server#1476

source/ada/lsp-ada_handlers.ads

@@ -428,6 +428,7 @@ private
    overriding function Project_Tree_Is_Aggregate (Self : Message_Handler)
      return Boolean is
      (Self.Project_Tree_Is_Defined
+      and then Self.Project_Tree.Root_Project.Is_Defined
       and then Self.Project_Tree.Root_Project.Kind in GPR2.Aggregate_Kind);
 
    overriding procedure Reload_Project (Self : in out Message_Handler);

testsuite/ada_lsp/hover.no_root_project/default.gpr

@@ -0,0 +1,6 @@
+--  Include a project that does not exist so that
+--  it fails to load in the ALS.
+with "unexisting";
+
+project Default is
+end Default;

testsuite/ada_lsp/hover.no_root_project/main.adb

@@ -0,0 +1,6 @@
+with Ada.Text_IO;
+
+procedure Main is
+begin
+  Ada.Text_IO.Put_Line ("Hello");
+end Main;

testsuite/ada_lsp/hover.no_root_project/test.json

@@ -0,0 +1,138 @@
+[
+   {
+      "comment": [
+         "This test checks that the textDocument/hover request does not crash ",
+         "even when the project could not be loaded properly."
+      ]
+   },
+   {
+      "start": {
+         "cmd": ["${ALS}"]
+      }
+   },
+   {
+      "send": {
+         "request": {
+            "jsonrpc": "2.0",
+            "id": 0,
+            "method": "initialize",
+            "params": {
+               "processId": 1,
+               "rootUri": "$URI{.}",
+               "capabilities": {}
+            }
+         },
+         "wait": [
+            {
+               "id": 0,
+               "result": {
+                  "capabilities": {
+                     "textDocumentSync": 2
+                  }
+               }
+            }
+         ]
+      }
+   },
+   {
+      "send": {
+         "request": {
+            "jsonrpc": "2.0",
+            "method": "initialized"
+         },
+         "wait": []
+      }
+   },
+   {
+      "send": {
+         "request": {
+            "jsonrpc": "2.0",
+            "method": "workspace/didChangeConfiguration",
+            "params": {
+               "settings": {
+                  "ada": {
+                     "projectFile": "$URI{default.gpr}",
+                     "scenarioVariables": {},
+                     "defaultCharset": "ISO-8859-1",
+                     "enableDiagnostics": false,
+                     "followSymlinks": false,
+                     "documentationStyle": "gnat",
+                     "namedNotationThreshold": 3,
+                     "foldComments": false
+                  }
+               }
+            }
+         },
+         "wait": []
+      }
+   },
+   {
+      "send": {
+         "request": {
+            "jsonrpc": "2.0",
+            "method": "textDocument/didOpen",
+            "params": {
+               "textDocument": {
+                  "uri": "$URI{main.adb}",
+                  "languageId": "Ada",
+                  "version": 0,
+                  "text": "with Ada.Text_IO;\n\nprocedure Main is\n\nbegin  Ada.Text_IO.Put_Line (\"Hello\");\nend Main;\n"
+               }
+            }
+         },
+         "wait": []
+      }
+   },
+   {
+      "send": {
+         "request": {
+            "jsonrpc": "2.0",
+            "id": 4,
+            "method": "textDocument/hover",
+            "params": {
+               "textDocument": {
+                  "uri": "$URI{main.adb}"
+               },
+               "position": {
+                  "line": 0,
+                  "character": 12
+               }
+            }
+         },
+         "wait": [
+            {
+               "id": 4,
+               "result": {
+                  "contents": [
+                     "<HAS>",
+                     {
+                        "language": "ada",
+                        "value": "package Ada.Text_IO"
+                     }
+                  ]
+               }
+            }
+         ]
+      }
+   },
+   {
+      "send": {
+         "request": {
+            "jsonrpc": "2.0",
+            "id": 7,
+            "method": "shutdown"
+         },
+         "wait": [
+            {
+               "id": 7,
+               "result": null
+            }
+         ]
+      }
+   },
+   {
+      "stop": {
+         "exit_code": 0
+      }
+   }
+]

testsuite/ada_lsp/hover.no_root_project/test.yaml

@@ -0,0 +1 @@
+title: 'hover.no_root_project'