# Buggy code
def preprocess(self, state, args, kwargs):
    rcc_files = kwargs.pop("qresources", [])
    if not isinstance(rcc_files, list):
        rcc_files = [rcc_files]
    ui_files = kwargs.pop("ui_files", [])
    if not isinstance(ui_files, list):
        ui_files = [ui_files]
    moc_headers = kwargs.pop("moc_headers", [])
    if not isinstance(moc_headers, list):
        moc_headers = [moc_headers]
    moc_sources = kwargs.pop("moc_sources", [])
    if not isinstance(moc_sources, list):
        moc_sources = [moc_sources]
    srctmp = kwargs.pop("sources", [])
    if not isinstance(srctmp, list):
        srctmp = [srctmp]
    sources = args[1:] + srctmp
    if len(rcc_files) > 0:
        rcc_kwargs = {
            "output": "@BASENAME@.cpp",
            "arguments": ["@INPUT@", "-o", "@OUTPUT@"],
        }
        rcc_gen = build.Generator([self.rcc], rcc_kwargs)
        rcc_output = build.GeneratedList(rcc_gen)
        qrc_deps = []
        for i in rcc_files:
            qrc_deps += self.parse_qrc(state, i)
        rcc_output.extra_depends = qrc_deps
        [rcc_output.add_file(os.path.join(state.subdir, a)) for a in rcc_files]
        sources.append(rcc_output)
    if len(ui_files) > 0:
        ui_kwargs = {
            "output": "ui_@BASENAME@.h",
            "arguments": ["-o", "@OUTPUT@", "@INPUT@"],
        }
        ui_gen = build.Generator([self.uic], ui_kwargs)
        ui_output = build.GeneratedList(ui_gen)
        [ui_output.add_file(os.path.join(state.subdir, a)) for a in ui_files]
        sources.append(ui_output)
    if len(moc_headers) > 0:
        moc_kwargs = {
            "output": "moc_@BASENAME@.cpp",
            "arguments": ["@INPUT@", "-o", "@OUTPUT@"],
        }
        moc_gen = build.Generator([self.moc], moc_kwargs)
        moc_output = build.GeneratedList(moc_gen)
        [moc_output.add_file(os.path.join(state.subdir, a)) for a in moc_headers]
        sources.append(moc_output)
    if len(moc_sources) > 0:
        moc_kwargs = {
            "output": "@BASENAME@.moc",
            "arguments": ["@INPUT@", "-o", "@OUTPUT@"],
        }
        moc_gen = build.Generator([self.moc], moc_kwargs)
        moc_output = build.GeneratedList(moc_gen)
        [moc_output.add_file(os.path.join(state.subdir, a)) for a in moc_sources]
        sources.append(moc_output)
    return sources


# Traceback (AttributeError)
Traceback (most recent call last):
File "/home/jenkins/workspace/meson/mesonbuild/mesonmain.py", line 282, in run
app.generate()
File "/home/jenkins/workspace/meson/mesonbuild/mesonmain.py", line 169, in generate
g.generate(intr)
File "/home/jenkins/workspace/meson/mesonbuild/backend/ninjabackend.py", line 184, in generate
self.generate_target(t, outfile)
File "/home/jenkins/workspace/meson/mesonbuild/backend/ninjabackend.py", line 257, in generate_target
self.generate_custom_generator_rules(target, outfile)
File "/home/jenkins/workspace/meson/mesonbuild/backend/ninjabackend.py", line 1360, in generate_custom_generator_rules
self.generate_genlist_for_target(genlist, target, outfile)
File "/home/jenkins/workspace/meson/mesonbuild/backend/ninjabackend.py", line 1406, in generate_genlist_for_target
elem.write(outfile)
File "/home/jenkins/workspace/meson/mesonbuild/backend/ninjabackend.py", line 116, in write
if quote_char == '"':
File "/home/jenkins/workspace/meson/mesonbuild/backend/ninjabackend.py", line 35, in ninja_quote
return text.replace(' ', '$ ').replace(':', '$:')
AttributeError: 'NoneType' object has no attribute 'replace'

# Fixed code
def preprocess(self, state, args, kwargs):
    rcc_files = kwargs.pop("qresources", [])
    if not isinstance(rcc_files, list):
        rcc_files = [rcc_files]
    ui_files = kwargs.pop("ui_files", [])
    if not isinstance(ui_files, list):
        ui_files = [ui_files]
    moc_headers = kwargs.pop("moc_headers", [])
    if not isinstance(moc_headers, list):
        moc_headers = [moc_headers]
    moc_sources = kwargs.pop("moc_sources", [])
    if not isinstance(moc_sources, list):
        moc_sources = [moc_sources]
    srctmp = kwargs.pop("sources", [])
    if not isinstance(srctmp, list):
        srctmp = [srctmp]
    sources = args[1:] + srctmp
    self._detect_tools(state.environment)
    err_msg = (
        "{0} sources specified and couldn't find {1}, "
        "please check your qt4 installation"
    )
    if len(moc_headers) + len(moc_sources) > 0 and not self.moc.found():
        raise MesonException(err_msg.format("MOC", "moc-qt4"))
    if len(rcc_files) > 0:
        if not self.rcc.found():
            raise MesonException(err_msg.format("RCC", "rcc-qt4"))
        qrc_deps = []
        for i in rcc_files:
            qrc_deps += self.parse_qrc(state, i)
        basename = os.path.split(rcc_files[0])[1]
        name = "qt4-" + basename.replace(".", "_")
        rcc_kwargs = {
            "input": rcc_files,
            "output": name + ".cpp",
            "command": [self.rcc, "-o", "@OUTPUT@", "@INPUT@"],
            "depend_files": qrc_deps,
        }
        res_target = build.CustomTarget(name, state.subdir, rcc_kwargs)
        sources.append(res_target)
    if len(ui_files) > 0:
        if not self.uic.found():
            raise MesonException(err_msg.format("UIC", "uic-qt4"))
        ui_kwargs = {
            "output": "ui_@BASENAME@.h",
            "arguments": ["-o", "@OUTPUT@", "@INPUT@"],
        }
        ui_gen = build.Generator([self.uic], ui_kwargs)
        ui_output = build.GeneratedList(ui_gen)
        [ui_output.add_file(os.path.join(state.subdir, a)) for a in ui_files]
        sources.append(ui_output)
    if len(moc_headers) > 0:
        moc_kwargs = {
            "output": "moc_@BASENAME@.cpp",
            "arguments": ["@INPUT@", "-o", "@OUTPUT@"],
        }
        moc_gen = build.Generator([self.moc], moc_kwargs)
        moc_output = build.GeneratedList(moc_gen)
        [moc_output.add_file(os.path.join(state.subdir, a)) for a in moc_headers]
        sources.append(moc_output)
    if len(moc_sources) > 0:
        moc_kwargs = {
            "output": "@BASENAME@.moc",
            "arguments": ["@INPUT@", "-o", "@OUTPUT@"],
        }
        moc_gen = build.Generator([self.moc], moc_kwargs)
        moc_output = build.GeneratedList(moc_gen)
        [moc_output.add_file(os.path.join(state.subdir, a)) for a in moc_sources]
        sources.append(moc_output)
    return sources