Multi-generators cannot be used without 'build_type' setting

conans/client/generators/cmake_find_package_multi.py

@@ -95,7 +95,7 @@ def filename(self):
     @property
     def content(self):
         ret = {}
-        build_type = self.conanfile.settings.get_safe("build_type")
+        build_type = str(self.conanfile.settings.build_type)
         build_type_suffix = "_{}".format(build_type.upper()) if build_type else ""
         for _, cpp_info in self.deps_build_info.dependencies:
             depname = cpp_info.get_name("cmake_find_package_multi")
@@ -110,9 +110,6 @@ def content(self):
                 format(version=cpp_info.version)
         return ret
 
-    def _build_type_suffix(self, build_type):
-        return
-
     def _find_for_dep(self, name, cpp_info):
         lines = []
         if cpp_info.public_deps:

conans/client/generators/visualstudio_multi.py

@@ -15,7 +15,7 @@ def __init__(self, settings):
             raise ConanException("Undefined Visual Studio version %s" %
                                  settings.get_safe("compiler.version"))
 
-        self._props = [("Configuration", settings.get_safe("build_type")),
+        self._props = [("Configuration", settings.build_type),
                        ("Platform", {'x86': 'Win32',
                                      'x86_64': 'x64'}.get(settings.get_safe("arch"))),
                        ("PlatformToolset", toolset)]

conans/test/functional/generators/multi_generators_test.py

@@ -0,0 +1,32 @@
+import textwrap
+import unittest
+
+from parameterized import parameterized
+
+from conans.test.utils.tools import TestClient
+
+
+class MultiGeneratorsTestCase(unittest.TestCase):
+
+    @parameterized.expand([("cmake_find_package_multi",),
+                           ("visual_studio_multi", ),
+                           ("cmake_multi", )])
+    def test_no_build_type_test(self, generator):
+        client = TestClient()
+
+        conanfile = textwrap.dedent("""
+            from conans import ConanFile, CMake
+
+            class Conan(ConanFile):
+                settings = "os", "arch", "compiler"
+                generators = "{generator}"
+
+                def build(self):
+                    cmake = CMake(self)
+                    cmake.configure()
+        """.format(generator=generator))
+
+        client.save({"conanfile.py": conanfile})
+        client.run('install . -s compiler="Visual Studio"'
+                   ' -s compiler.version=15 -s compiler.toolset=v100', assert_error=True)
+        self.assertIn("ERROR: 'settings.build_type' doesn't exist", client.out)