Issue #25144: Ensures TargetDir is set before continuing with custom install.

Misc/NEWS

@@ -333,6 +333,9 @@ Build
 Windows
 -------
 
+- Issue #25144: Ensures TargetDir is set before continuing with custom
+  install.
+
 - Issue #1602: Windows console doesn't input or print Unicode (PEP 528)
 
 - Issue #27781: Change file system encoding on Windows to UTF-8 (PEP 529)

Tools/msi/bundle/bootstrap/PythonBootstrapperApplication.cpp

@@ -323,6 +323,9 @@ class PythonBootstrapperApplication : public CBalBaseBootstrapperApplication {
         case ID_CUSTOM_INSTALL_BUTTON:
             SavePageSettings();
 
+            hr = EnsureTargetDir();
+            ExitOnFailure(hr, L"Failed to set TargetDir");
+
             hr = BalGetStringVariable(L"TargetDir", &targetDir);
             if (SUCCEEDED(hr)) {
                 // TODO: Check whether directory exists and contains another installation